And, it seems an upstream fix commit 979923871f69 ("x86/timer: Don't skip PIT setup when APIC is disabled or in legacy mode") has been backported to v5.4.19 and therefore focal kernel included that in bug 1863588 since at least 5.4.0-15. So it should be fine for you to use focal kernel now.
Or, maybe you don't bother upgrade kernels from Focal. Just use 5.3 kernels from Bionic, as they should have the same fix since 5.3.0-46.
@Anthony, you can try focal kernel directly on your Bionic installation first.
$ printf "deb http:// archive. ubuntu. com/ubuntu/ focal main restricted universe multiverse\ndeb http:// archive. ubuntu. com/ubuntu/ focal-updates main restricted universe multiverse\n" > /etc/apt/ sources. list.d/ focal.list extra-5. 4.0-26- generic linux-firmware/ focal
$ sudo apt update
$ sudo apt install linux-modules-
And, it seems an upstream fix commit 979923871f69 ("x86/timer: Don't skip PIT setup when APIC is disabled or in legacy mode") has been backported to v5.4.19 and therefore focal kernel included that in bug 1863588 since at least 5.4.0-15. So it should be fine for you to use focal kernel now.
Or, maybe you don't bother upgrade kernels from Focal. Just use 5.3 kernels from Bionic, as they should have the same fix since 5.3.0-46.