OK, what do you mean by “’#’ x 1500 though”? 1500x1500? To support 1440p screens?
1GB of sprite doesn’t look that much… or you’re talking just about characters, and FX and backgrounds add much more? (I felt like characters were the biggest, but it’s true that FX frames can quickly add up).
On my Renpy project I encore most sounds to .opus and pictures to .webp max quality for high quality and low space usage. However, Renpy uses assets directly. Unity compresses them with its own algo, so I’m not sure it will make a difference in build. Import settings, esp. format, and texture packing settings probably make the biggest difference. But I’ve never worked on a game with so many assets (except in an indie studio that was making a HD 2D game with many backgrounds and detailed props at 4K res - probably the 2D game that took the most space I have ever installed on Steam!).
It is possible that Unity tries too hard to keep layers on the PSD, though. I would imagine that it renders to a flat picture on build, but I’m not sure. You can try and export some of the biggest PSD to a flat format (png or webp) and then see how it affects build size.
For the battle FX I can’t tell, but you can evaluate their size in build by building a sample project just with them. Same remark for your PSD / flat PNG. Assuming they are packed on different spritesheets so there is no interaction between them.