Only day one for iPhone here. Same for watch. Mac when I need, based on rumors tho, if I needed a new Mac now I wouldn’t buy, but if I needed one 6 months ago I would have bought it.
But for iOS devices for me, strictly day one only
The refresh was just dropping the base 128GB configuration. This made the 256GB model the new base. I'm not aware of any changes made to the mini since 2018
When it comes to iPhone or Apple Watch I would say six months after launch unless my current phone needs replacing. If it's less than six months to the new version my logic is to just wait.
When it comes to stuff like Mac computers unless I hear about a big change coming up like the ARM transition or some specific change like the keyboard in MacBooks I don't see any reason plan purchases around release times.