MTG Salvation

Sun Titan/Kamiz, Obscura Oculus

If I arrange the triggers, resolving Kamiz's connive trigger before Sun Titan's trigger, is it possible for Sun Titan to reanimate whatever I discarded, or does Sun Titan needs to have a target right as its trigger goes to the stack?

The latter. As you put Sun Titan's triggered ability on the stack you must target a "permanent card with mana value 3 or less [in] your graveyard" right then and there (C.R. 603.3d, 601.2c), regardless of the order in which that ability and Kamiz's ability are put on the stack. Likewise, you must target an "attacking creature" as you put Kamiz's ability on the stack.

So it doesn't matter if the Sun Taitan clause is a "may". I remember asking a similar question regarding Land Tax and Opposition Agent about the searching, but LT doesn't have a target requirement.

The "may" is an option given at resolution of the spell/ability. It is then that you choose to take that option or not. But targets are chosen when the spell/ability is put on the stack, "may" or not. You do not choose to put a "may" trigger on the stack or not, you have no choice in that matter, you have to put it on the stack, with modes, targets etc. selected as required.

In general, choosing the required number of targets for a spell or ability is not optional, even if it contains "you may" in its text (C.R. 603.5, 603.3d, 601.2c). See also this thread and this thread.