Thanks! Would not have looked there without your pointer..
Apparently it’s currently set to “None”
Should I change this to “Import/Persistent Auto-generated” to use my own?
(It seems to be using my certificate anyway)
Yes, otherwise it will generate a new one on restarts.
Installed 384.3 on my AC68U (upgraded from 380.69_2) and even after performing a factory reset I think I'm seeing some lag on both 2.4 and 5 GHz compared to the older generation (Apple devices mostly).
Disable Airtime Fairness and Implicit Beamforming. Forget the SSID on your mobile devices and reconnect them.
Are you going to add support for Let's Encrypt certificate issuing and renewal from the DDNS tab, also for RT-AC3200?
Unknown yet. It's closed source, so I cannot add it to every models.
Can I update straight from there to the new Merlin 384.3 build or do I need to jump to some intermediary build before that?
378.55 should be fine, be prepared to do a factory default reset after flashing.