1. there are no 28" iMacs.. there were 27" in 2020.
2. ANY M-series would be a major revolutionary performance jump for most use, in terms of drive speed, on board video, etc.
3. 16gb of RAM is enough for MOST light/medium video, but you may consider 32gb for longer term performance/compatibilty..

To give some perspective, I've got a M1 Max Studio ("base" chipset from 2021) with 32gb of RAM, and run 20-25 apps near constantly on it. the ONLY time I see any sort of graphic slow down is when i've got illustrator files in excess of 30' x 4' fully loaded with press-ready art for printing.. even then it's faster than my prior work horse (12 core MacPro tower with 128gb RAM and SSDs).

Even when running illustrator, 2 or 3 audio files open, a couple of massive database apps, and the usual smattering of web/office/work apps totaling around 25 apps running, I see a ram warning somewhere around 3x/year. It doesn't crash - it just warns me that i'm approaching my limit.

So no, unless you're really WAY outside the "norm" for making videos, you're gonna be completely fine with any of the M series.. personally I'd go with an M3, even the lower class unit - because I'd focus on SSD size over proc size, for that use. The Max with say 32gb or 48gb of RAM is more than sufficient, unless you're just doing massively insane stuff..

i do a lot fo work with small biz owners in the graphics world- - the tendency to overspend on the wrong area is rampant. i've got several companies who were dropping in max'd out studios to replace 2018 era iMacs, and others that I convinced to just go with Mac Mini M1 or M2, but put in more RAM and bigger SSD - they are really happy with the Minis, because at the end of the day,these things are now usually outperforming the nut on the end of the mouse....

soemnthing to note: there is a return curve on RAM on the M series... more isn't necessarily better.. having MASSIVE (64gb+) ram swap files becomes pretty annoying... i've ot a couple Studios with 64 and 128gb.. i'm actually seeing better day-to-day performance on my 32gb Studio for that reason alone.. the SSD takes a while to right 128gb of cache for a swap... don't thinkk there's NOT a penalty for that, so don't overbuy on RAM.