The obvious point here is that Apple would never release two different iMacs in the same year. The first AS Mac will be a MacBook of some sort.
I agree. While the small iMac did not get it's update yet, a ARM release with new design would clearly hurt the Intel 27 incher in it's sales by showing of the future new design and other gimmicks. ARM iMac will probably hit February or march.
I assume next will be the bigger, more powerful, MacBooks and after that it's 27 inch iMacs turn before eventually MacPro and iMac Pro (if they keep it around) follow. They sort of need drip feed the "power levels" to the consumer so they can get exited about things to come (like right now, with possible GPUs, comparability and such). 27 inch will sell better if you can say "twice aspowerfull as the small one", MacPro will sell better if you can say "4 to 20 (if you trade in one of your houses for it) times as powerful as iMac 27 inch".
Also for the more pro machines it obviously helps if software has some time to come around and be optimized while the first years also might reveal some hardware things to be fixed on apples side of things.
I doubt this will go much quicker than the two years they announced.
If Apples makes the MacBook Pro (meaning not portliness) again i might be in the marked for a more recent 13 incher in a few years. Otherwise i will probably just hope that my final cut will automatically come with the iPad Pro App and just switch to a iPad Pro from my 2018 iPad for the time being.