Not sure if it's my laptop (Intel AC 9260) but 2.4GHz WLAN being roughly as fast as 5GHz pretty much no matter the card and router used as AP is pretty damn weird and it doesn't seems to be channel interference. (And Ethernet is way faster)
Maybe I'll just try to pull an Ethernet cable through the walls to have Ethernet upstairs even if I'm renting this place. After all fiber got in my flat by shoving+pulling it in the conduits originally made for the cable Tv.