I fully disagree with the overly simplistic buy when you need it claim. Unless a need is urgent (such as if one's old box failed) then it makes total sense to buy based on evaluation of the timing of the expected coming new generations. I waited for M2, for instance, because I wanted what I (correctly) thought was coming with M2.Buy what you need when you need it. Don´t speculate on anything which might be happening in the future - be it hardware changes or software support.
We can´t help you chase discounts, though.
The down side of waiting is that we really do not know the timing of what Apple will do. I wanted an M2 Studio Max, for instance, but Apple delayed so long that I ran out of patience with the limiting RAM of my old box and instead ended up with an M2 MBP Max.
So IMO the OP's thought processes are sound. Unfortunately it will always be a best guess kind of thing as to timing. But a Mini is not a Mac Pro. Most folks do not expect any major changes to the Mini, just an evolutionary 10%-20% performance increase. IMO with a Mini it would be more important to get a maximum configuration, especially RAM, than it would be to get M3 over M2.