I intend to use this to play with as many planets as possible. Should I expect a noticeable amount of lag toward the mid-late game, or is it fine?

You might want to tweak some of the startup settings, specifically for Tiber and possibly Cerys (if they haven't already updated to fix those issues), according to this performance thread, but aside from those, there shouldn't be any performance issues.

there is a map setting to stop tiberium from growing ^^ That made a huge difference

I've been running with GTTS (Global Tick Time Scale) set to 30 UPS for a while now, because my 5700X3D could not keep up with only 1/2 (now 1/3) the planets colonized. Other than Rubia (which got an update to mostly fix it), every planet so far has been behaving with this. It's been a game saver, considering.