It seems the problem could be tied to Kerberos encryption. Your workarounds are solid, though they come with their own quirks. Upgrading Samba to 4.16 sounds promising, but the version clashes are a bummer. And tweaking local security policies might help partially.
As for upgrading Samba to 4.16 on Ubuntu, it's a bit of a challenge due to the version compatibility. You might want to explore Ubuntu 22.04, which might have better compatibility.
On a side note, you might find some valuable insights on the
cheap Windows 10 key Reddit sub. They occasionally share useful tips that could align with your situation. Hopefully, a combination of technical fixes and community advice can get your setup back on track.