Another run of memtester was completed with no issues on Saturday, but I forgot to report back. I connected to it wirelessly, did the upgrade to 20220528-84fdbeb on Sunday and nothing else for that day.
And today, I started the day (Monday 30th of May, ~7am) with testing. I got the rpi next to me again, powered it on and even after 2 minutes it was not visible anywhere. Pinging its .local name (I use zeroconf everywhere on my network) returned that the host is unreachable. It was not visible under the router's connected clients too, so I unplugged and replugged it.
Same thing happened after the reboot, so I had another idea. Leaving the rpi on the same spot, I tried the sdcard with lakka on. It booted with no issues, connected wirelessly and I could see it under connected clients. I ssh-ed to it and shut it down properly.
Then I got an ethernet cable and connected it to my router, hoping to get a reliable wired connection. It showed up for a few seconds under connected clients and it suddenly dissappeared. Disconnecting the end of the cable from the rpi's end only and reconnecting it gave a stable wired connection. I ssh-ed to it to run ifconfig -a only and saw that the wireless was completely disconnected!
I sweared a bit and moved on for today's upgrade (20220530-2d50f5f). I downloaded the tar file, checked its sha256sum, it was correct so I rebooted. And after the reboot it was still on yesterday's version...
I connected ssh-ed to it again to start over and while it was downloading the update file again, it got disconnected and reconnected with a different ip! First time on wired it was 192.168.1.142, second time was .143 and third time it was .144.
I know from experience that *elecs request a new ip from the dhcp server although their lease time has not passed (I have mine set to 1 day), so the above is now normal behavior.
The file was downloaded completely, its checksum was checked again, it was correct again, but no update happened again. I am now downloading it for the third time and I will give up if it fails again.
---edit
Oh look it worked! And with that third try, I am now on 20220530-2d50f5f after 30+ minutes of trying something that would take 5 at most.