And here I am struggling to get gigabit Ethernet over brand new cat6 cabling with devices bought in 2025…

Over what distance? Sounds like something is broken… Sure all the pairs are punched down in the right order, cables aren’t crushed, etc.?

I’ve had 2.5GbE running over old in-wall Cat5 in old buildings on short to medium (15-30m) runs with no issues…

	▲	maccard 9 hours ago \| parent [-]
		Roughly 15 metres. Almost certainly a cabling issue, although frustratingly the electrician who installed it says that they think it's fine. I've got a ubiquiti U7 AP connected to a USW lite 8 via POE, and it negotiates GbE, but then falls back to E.