I went though more or less this same process.
Unfortunately I don't know if there is a good solution to the IMEI issue. but I stopped looking into it because I did finally find the solution for the UT update (the device is actively supported and 16.04 OTA-25 and 20.04 dev are available). There are just an unfortunate pairing of bugs that causes it to be a real pain to setup the device.
I added my best shot at documenting the problem/solution on the issue on the begonia gitlab page
As far as the IMEI goes, my suspicion is that the UB ports installer version for the note 8 uses android 9 to get around the adb issues that the note has in halium 10, and from the interwebs there seemed to be some indication that the android 9 formatting doesn't line up with the android 10 (or 11) firmware. If this is true there could be the chance that after getting the note updated to a current version of UT you may be able to re-install miui without IMEI issues.