Sorry, I should have made my homework...
irqbalance was installed but not enabled (something I regularly forget on new OpenWrt installs). With irqbalance and just cake on eth1+2, latency indeed looks great:
So back to the EAP245v3. It only supports 802.11ac, the previous plot was ac as well. It's still slow, but at least it looks less chaotic.
I don't know what happened after 250s, I had to reconect, but dmesg on the ap didn't reveal anything. This was right after a reboot. I'll test again after a few hours/days uptime, since wifi tends to get worse after a few days.