No. You only need to make other variables in the UMG if you actually need that.
My game has A LOT in the save game. Nothing copied into the UMG.
If you can already make materials change, you can write to the save game when you do that.
If you’re changing the materials from the UMG, then it might be logical to do it there, but it really doesn’t matter.
If you get stuck, come back here, and I’ll show you how…