You will not feel a difference in coding/dev work between M1 to M6 in a relevant manner.
I have an M1 and M4 Max at office. I feel the sluggishness of M1 now, after years it finally started to show its age.
Have been considering a used M1 but I'm concerned that I'm going to get flash storage that's been thrashed to within an inch of it's life by someone running it as a poorly configured inference host.
Yeah, I know. The problem is that used prices right now are almost the same as what they used to cost brand new. Prices have gone up way too much.
I also can feel a speed/sluggishness difference between my M1 Max MacBook Pro and my M4 Pro (14 core).
Two years ago maybe, when I wrote code by hand.
Today development work is limited by compile times and test run times, particularly with UI tests in the iOS simulator.
Hardware on the development machine finally maters.