Procreate can handle a 4096x4096 canvas with 1GB RAM.
I wonder what it can do with 4!
Re OS X on ARM: not in any foreseeable future (apps would stop running, too much developer nightmare for too little benefit, and we already have an ARM OS). But the name being "macOS" is a sure bet. And in the VERY long run, anything is possible, including OS X on some descendent of today's ARM. Even ARM will go away... someday.