Similar setup (not in a 1U). I have 3 Pi 5's in my AV rack with M2 and PoE Hats - they actually work fairly well and bonus? Don't need to power them. I could get a 1U enclosure but, it's fine.

The only issue is one of the PoE Hats fan is catching something (though nothing i can see), so on occasion it will need persuasion to be quiet