I think most of that is really opinion and experiences. No doubt it’s not designed or built truly for racks but folks have been making rack mounts for Mac minis since they first came out.

On the upgrade path I don’t think upgrades are truly a thing these days. Aside from storage for most components by the time you get to whatever your next cycle is, it’s usually best/easiest to refresh the whole system unless you underbought the first time around.