Thanks to Calvin, Frank and Jack.

I'll use shallow 2" boxes for anything that doesn't need too much wire
spare (switches for example). For anything else that has two or more
cables coming in, Calvin mentioned that the technique I used of
putting a ring over a 4" box and plastering over the ring should be
okay per the CEC. I'm in Quebec and we are technically not allowed to
do our own wiring here, so I won't be able to have it inspected
anyway.

Interior walls will be 2x4 so they will have standard boxes. I'll plan
my circuits to go through these walls first so that a GFCI can be
installed upstream if needed.

Thanks everyone!