Debian on Latitude 5420

There are boot issues after installing either Buster or new Bullseye. It seems to do with Intel microcode.

Has anyone gotten Debian to work on this model? Any tricks?

Some details below.

1. Both Buster (10.10) and Bullseye install DVD is able to complete installation.
2. But after shutdown and startup, system goes into endless hang. Messages like watchdog: ;BUG: soft lockup - CPU#0 stuck for 23s etc come on screen.
3. Only way out is to hold power button until machine switches off.
4. Live CD of bullseye has exact same issue.

However, if I boot again from install DVD (both Buster and Bullseye) and go into rescue mode, seek a / prompt and just reboot, it boots fine!

Which means a cold boot has some issue with Intel microcode (my guess).

I tried disabling microcode update using dis_ucode_ldr but it doesn't seem to make a difference.
I originally attributed issue to iwlwifi but removing it doesn't help. I did fresh install without any iwlwifi.
My CPU info shows family 6, i5-1145G7, Stepping 1, microcode 0x88.
After a successful boot using rescue described above, journalctl shows microcode as sg: 0x806c1, pf=0x80 revision: 0x86.