I am happy to report I have solved my issue. As I originally suspected, some sort of QoS settings were likely stuck on even though it wad disabled (the QoS option finally disappeared). Probably some sort of bug. Hard reset combined with July firmware has me back to normal. I'm too nervous to re-update to the latest FW in case that's what ruined it, but at least I know how to fix it now if it happens again - I don't know for sure if it was the FW or if the thing just needed a hard reset after 5 years of continuous use haha, but I'm happy to have it fixed. 180 Mbps on all devices, 2 floors up from the router and in every corner of the house