I'm not someone who buys boosters or boxes (I have been known to draft on occasion though), but I'm interested in playing Brawl. I like commander, but hate that every deck has the same 5 or 6 artifacts, so it'll be nice to play a version that isn't built around whether you draw your Sol Ring and your Opponents don't.
The correct answer is you need to have the right number of colored mana sources to hit your colors on time, and then all other lands can be colorless/utility lands. This is a good guide to how many colored mana sources to hit your colors.
Now, the type of deck you're playing will dictate what lands you can get away with. For example, if you're playing control, you can get away with far more ETB Tapped lands than if you're playing an aggro deck.
In your deck, and using the chart in that article, you need 19 black mana sources to consistently play Gifted Aetherborn on turn 2 (your lowest cost BB spell). You're running 24 total lands (I would probably run 25 since your deck is running Liliana and Tetzimoc), so since you only need 19 colored sources, you can have 5 colorless lands without impacting your color need.
There's no denying desert control is an effective mill strategy. My only issue with it is it's SUPER slow lol. In a competitive match, you're looking at going to time most of the time.
So far the best (as in win the most games) Mill decks I've seen are the desert control decks, like Bant or Sultai deserts. They normally have a main win condition, like Approach, but it seems like half the time they get the mill win. The combo mill decks are too inconsistent (since you pretty much have to have Fraying + Sanity AND get an attack in) and the turbo mill plan just isn't turbo enough.
Have you considered going UB and then lightly splashing red for Beckett Brass? I think that would be easier on the mana base and probably a much more stable deck overall.
Building on what D90Dennis14 is saying, I think UR/x Drake is the deck you want to be playing. It does what you want to do, and it plays better creatures than the ones you're running. As a matter of fact, most of their suggestions seem like they're coming from that deck/thread lol
I realized that about Chart a Course vs Tormenting Voice when I suggested it. It's still strictly better. drawing then discarding is almost always going to be better than discarding and then drawing. For 1, you have the most information, so if you do something like draw crap cards, you can just pitch one of those. With Tormenting, you run the risk of having to toss better cards than you're getting. For 2, you can still cast it when you top deck it empty handed. If you topdeck Tormenting without cards in hand, you have to wait until next turn to cast. It's not fun.
Don't get me wrong, pre-rotation, I played UR Zombie Emerge, and Voice/Reunion were a big part of it. Still love that deck! But, Chart a Course would've been just flat out better.
I too am tinkering around with Colossus and Sunbird's in a fun deck. I'm not sure where I'm gonna go for it though.
Your deck seems to have too few lands.
Pirate's Pillage is very bad. Like really really bad. I also don't really care for Tormenting Voice or Cathartic Reunion in this list. Since you're playing blue anyway, might I suggest Chart a Course? If you aren't playing a GY based deck (and you aren't), it's strictly better than Tormenting Voice.
You should also look into adding Key to the City. It can help sift through the deck. It can make your Colossus unblockable. And if you tap it as part of an Improvise cost, you can just draw 2 having never discarded!
Kinjalli's Sunwing doesn't flip Path of Mettle, so it's probably a sideboard card. I think I would run Bomat Courier it's it's place.
I'd also probably move Magma Spray to the sideboard for more creatures. Probably Ahn-Crop Crasher.
Have you considered Dusk // Dawn against Dinos? It kills theirs without giving them a bunch of lands. Plus you have a few creatures that would survive it. I use it in my deck against Dinos, and it works great. I also have more creatures that slip under it, though.
Finally, you have a lot of 5 drops for 23 lands. I think, for 5 drops, you want to be at 25 lands. 24 lands at the minimum, and even that's pushing it.
Personally, I think when it comes to Cast Out/Binding, it's the right move to have a mix. You don't want to be all in on Cast Out or all in on Binding, just to get Binding'd. I've done it to opponents, and seen their faces sink. Also, if you can at all help it, never be the first to Binding.
Do you run into a ton of Gearhulks? I don't think I'd dedicate a sideboard card to removing them specifically. There's plenty of creature kill/artifact hate in black and white that I don't think you have to worry too much about them. In that spot, I'd run something simple like Forsake the Worldly instead. It's a great sideboard card. Permenantly gets rid of artifacts and enchantments and cycles away if you draw it and don't need it.
Instead of Gideon in the Approach match, I think I'd use Lost Legacy. Gideon sits there until your opponent can deal with him, then they Approach. It buys time, and sometimes that's enough, but I'd rather just cast the single Legacy and be done with Approach for the rest of the game. Best time to Approach is after they cast the 1st Approach. They normally won't tap out before that. Not saying they won't, just saying there's plenty of time, so you can be patient.
I think Gifted Aetherborn is good enough to be a main deck 4-of. Lots of aggro in the meta. It's the perfect card to fight those decks.
4th Sanctum Seeker is definitely better than Bishop. More impactful and lowers the curve.
https://www.channelfireball.com/articles/frank-analysis-how-many-colored-mana-sources-do-you-need-to-consistently-cast-your-spells/
Now, the type of deck you're playing will dictate what lands you can get away with. For example, if you're playing control, you can get away with far more ETB Tapped lands than if you're playing an aggro deck.
In your deck, and using the chart in that article, you need 19 black mana sources to consistently play Gifted Aetherborn on turn 2 (your lowest cost BB spell). You're running 24 total lands (I would probably run 25 since your deck is running Liliana and Tetzimoc), so since you only need 19 colored sources, you can have 5 colorless lands without impacting your color need.
Don't get me wrong, pre-rotation, I played UR Zombie Emerge, and Voice/Reunion were a big part of it. Still love that deck! But, Chart a Course would've been just flat out better.
Your deck seems to have too few lands.
Pirate's Pillage is very bad. Like really really bad. I also don't really care for Tormenting Voice or Cathartic Reunion in this list. Since you're playing blue anyway, might I suggest Chart a Course? If you aren't playing a GY based deck (and you aren't), it's strictly better than Tormenting Voice.
You should also look into adding Key to the City. It can help sift through the deck. It can make your Colossus unblockable. And if you tap it as part of an Improvise cost, you can just draw 2 having never discarded!
I'd also probably move Magma Spray to the sideboard for more creatures. Probably Ahn-Crop Crasher.
Have you considered Dusk // Dawn against Dinos? It kills theirs without giving them a bunch of lands. Plus you have a few creatures that would survive it. I use it in my deck against Dinos, and it works great. I also have more creatures that slip under it, though.
Finally, you have a lot of 5 drops for 23 lands. I think, for 5 drops, you want to be at 25 lands. 24 lands at the minimum, and even that's pushing it.
There's not a lot UR can do about Carnage Tyrant. You can Commit // Memory the spell on the stack, but that's about it.
Do you run into a ton of Gearhulks? I don't think I'd dedicate a sideboard card to removing them specifically. There's plenty of creature kill/artifact hate in black and white that I don't think you have to worry too much about them. In that spot, I'd run something simple like Forsake the Worldly instead. It's a great sideboard card. Permenantly gets rid of artifacts and enchantments and cycles away if you draw it and don't need it.
Instead of Gideon in the Approach match, I think I'd use Lost Legacy. Gideon sits there until your opponent can deal with him, then they Approach. It buys time, and sometimes that's enough, but I'd rather just cast the single Legacy and be done with Approach for the rest of the game. Best time to Approach is after they cast the 1st Approach. They normally won't tap out before that. Not saying they won't, just saying there's plenty of time, so you can be patient.
I think Gifted Aetherborn is good enough to be a main deck 4-of. Lots of aggro in the meta. It's the perfect card to fight those decks.
4th Sanctum Seeker is definitely better than Bishop. More impactful and lowers the curve.