So, I went and flashed US unlocked 20E last night. The directions started going sideways at step 5, setting active to slot b. I first got the android logo with the red ! as I recall but lost all fastboot control. Eventually, it booted back up into PIE.
I tried again to set it back to slot b, I got stuck and could only get into download mode. After applying all the LGUP patches, it recognized my phone, and I figured I'd play it safe and do a full factory flash, which was successful. The good news is that the bluetooth bugs I've been seeing with US Cellular 20A have disappeared, but I'm starting to wish I had root again.
I started following
Markus's TWRP flashing instructions, but when I run an "adb reboot bootloader" command, it just boots back into PIE. His instructions for forcing recovery by rebooting + usb connection + holding vol up or down don't seem to work for me. If I force a bootloader boot (power and vol down, press and release power at the LG logo screen), I get to what I think is the plain stock bootloader (wipe phone - yes or no, reboot) but I still have no longer fastboot controls from the command prompt. Is it possible that the US unlocked version bootloader is more limited than the US Cellular version?
To answer one of your questions about encryption, the SD card and device encryption were separate but only the SD card was reversible. If you encrypted your phone, the only way to turn it off is with a wipe. For example, I was running fully encrypted, and I decrypted my SD card no problem before I started everything last night figuring that I might be flashing TWRP and Magisk from it at some point along the process.
Interestingly, it seems that the encryption in US Unlocked 20E is also more limited than US Cellular's 20A version. It doesn't appear that you can encrypt the phone itself but just permit it to set a Pin/password before boot. On the US Cellular 20A version, there was a whole process of encrypting the phone once you turned it on. It also doesn't appear that you can totally decrypt your SD Card once you encrypt it on 20E. You are just able to turn off encryption for new files I think? I'm still playing around with it.