Realistically, going down to 12GB System RAM from 15GB is not very noticeable compared to going from 1GB to 4GB VRAM and so I think it should be preferred. If you ran into issues with a very RAM heavy game (unlikely), you could change it to 14GB + 2GB, or go all the way back to 15GB + 1GB. If you are using the Steam Deck on an external display that is higher resolution than its native 1280x800 display, you should probably do 12GB + 4GB to make room for higher resolution textures. Haven't tested this, but if there were noticable differences, I would think these are the most likely scenarios for them to crop up.
For the record, you don't have to take "everything else" out first. It's actually quite accessible once removing the Steam Deck back plate, which is easily done with a Phillips head screwdriver. The bulk of the 2-4 hour estimate on iFixit is dealing with the battery adhesive. (source: I've opened my Steam Deck to swap the SSD, and I just opened it to attempt a band-aid fix to the right bumper after dropping the Deck directly on it while waiting for the part to restock)