I don't pretend to know better market strategies and reasoning than Apple do themselves, but there are serious constraints placed on them by spending patterns and the dynamics of the consumer market.
They don't make money out of a user who buys a product and then never replaces it with a new one, so they have to have product development cycles, and new products therefore entering the market, but they wouldn't be able to routinely lead with high-end devices because there would not be enough income in that revenue stream, and it would (I think) tend to suppress sales of the lower-end models when they came out, with user expectations having been raised to a point above (or near the top end) performance of the high-end product.
This is why, I believe, they have led the M1 rollout with the MBA/MBP and 24-inch iMac. The interest and buyer commitment in that end of the market drives a lot of sales, makes a great deal of revenue, and that is what provides the foundation for their more esoteric product offerings to come.
I am also pretty certain that there will be good differentiation between the current M1 iMac and the supposedly 'pro' model to come, but history would suggest the 'Pro' model will be a few months off yet, so the 24-inch market settles in first.
As to your last point, indeed, it isn't a certainty that there will be a new larger iMac. My personal preference would be much as you suggest - an 'M1 Powermac', of similar form to a tall Mac mini, or short Mac Pro, and which would sit in the middle of their desktop market. One way or another, the next few Mac-related product rollouts are going to be quite interesting!
