Hello herewego and welcome to the ArtRage forums 
First I have to say right of the bat, I don’t know why your files are coming in so big. Sorry!
Secondly, that’s not a typical workflow that’s generally needed in AR in my experience.
I’ve used AR for many years (I think a late v2 is the oldest version I have) and I typically work at around 3500x4900px. And maybe ending up with 20 or so layers sometimes.
But my finished file sizes have, as far as I can recall, rarely gone much above 500MB or so.
You should be able with your system to do the entire painting start to finish in the one file I would have thought.
Hopefully someone from development will be able to chip in here…
Maker Of Replica Macoys
Techie Stuff:
ArtRage 6.1.1 ~ 15" Macbook Pro ~ macOS 10.14.6 ~ 4 Core i7 3.1GHz CPU ~ 16GB RAM ~ Wacom Intuos4 M