*buntu 20.04 freezes at/after login screen, Lenovo ThinkCentre M920x

Thread of me diagnosing this here: https://askubuntu.com/questions/1249081/xubuntu-20-04-freezes-at-after-login-screen-multiple-reinstalls-other-weird-is

In short: upgraded from working xubuntu 19.10 to 20.04, after which the system freezes at or just after the login screen MOST times. Not always. Sometimes it gets in and works fine for a while or continually. I reinstalled xubuntu and ubuntu 20.04 multiple times, same result. Ran all hardware diagnostics, all clean. Windows 10 dual boot is fine. Finally reinstalled xubuntu19.10 and that's working flawlessly.

Note my debug info is from 19.10 and therefore the OS info will be wrong, but the hardware is the same, which may help. Given how the liveUSB froze with similar behaviour I could potentially do diagnostics there if people have ideas for how to do so, though I don't know whether any logs will get saved to the USB stick so that I can extract them when booted back into 19.10?

Thanks in advance.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: ubuntu-release-upgrader-core 1:19.10.15.4
ProcVersionSignature: Ubuntu 5.3.0-59.53-generic 5.3.18
Uname: Linux 5.3.0-59-generic x86_64
ApportVersion: 2.20.11-0ubuntu8.9
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: XFCE
Date: Thu Jun 11 10:14:08 2020
InstallationDate: Installed on 2020-06-11 (0 days ago)
InstallationMedia: Xubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: release-upgrade
UpgradeStatus: No upgrade log present (probably fresh install)