I think you're right; the transition to ARM, if it happens, will be pushed to light duty MacBook first. I don't see it on desktop for at least a year but more likely 2+ if it even happens. Makes it tricky, I mean do you hold out to be ahead when/if the transition occurs or put your cash down on 14nM intel now knowing you are going to be on the outer in a couple of years?
I see spec bumps and possibly a move to 23" (from 21.5') now, with a possible iMac Pro redo Q4 (December release) with maybe miniLED on that machine. ARM MacBook Air Q2 2021, then, maybe, a WWDC 2021 announcement of new consumer iMac with trickle-down tech from the new iMP. All pure speculation of course...