The primary synergy with Toshi is instants - without those, Toshiro does nothing, and he needs nothing else to have synergy.
Then you have stuff that supports that synergy but doesn't have synergy on its own, such as self-mill to get instants into your grave without casting them, or non-instant removal, which can trigger toshiro. So that's the second degree of synergy.
Once I had the self-mill, suddenly self-recurring creatures like reassembling skeleton were better. Now we're up to three degrees of synergy.
And once I had a bunch of self-recurring creatures, I needed something to do with them to get decent value besides just chump blocking. So then I threw in grave pact and dictate of erebos and the like. Holy crap, we're up to four degrees.
And finally, now that I've got a grave pact engine going, I need sac outlets to reliably trigger them instead of relying on blocks. Five degrees of synergy at this point. Yikes.
You can see where this went off the rails. By the end, there were a lot of cards that had no direct synergy with my commander, and I was spreading myself way too thin across many different functionalities of cards. The odds of drawing cards that didn't work well together was way too high, and a lot of my cards were very low-impact without synergies.
So I think it's worth analyzing card choices in this way. Think to yourself - "why is this card in my deck, what makes it good here?" And if you find that it takes quite a few degrees of synergy - by which I usually mean more than about 3 at absolute maximum - then you should probably reevaluate and try to refocus your deck.