Cannot login after 10.15.3 update

I ended up solving all the issues that I was experiencing by doing a clean install of Catalina. Here's the steps I took:

1. Boot up from external drive that I have a working install of Mojave on (USB HD), a bootable USD thumb drive works too
2. Downloaded the latest Catalina installer from the app store (10.15.3)
3. Using disk utility, I erased the 2 partitions on the internal drive (Macintosh HD, and Macintosh HD - Data)
4. Launched the Catalina Installer and selected the internal drive as the destination for installation
5. Let the installer finish and completed the initial setup items
6. Now have a functional system again

This fixed many issues that had carried over from my Mojave install on this same internal drive. Quirkiness with Apple ID needing to entered numerous times and often not working, fixed an otherwise unusable install of Safari, fixed access to folders that mysteriously changed during Mojave updates, and finally fixed the inability to even login and use my iMac. Being that I have been a mac user since getting my first mac (Mac IIsi) back in about 1989, I have been used to being able to update without having to jump through too many hoops. With OSX, certainly most of the updates have gone very smoothly, Mojave and Catalina have been the exception to the rule. Every update broke something and took days to recover from. If this is the new norm, I would highly recommend backing everything up to an external drive, make a bootable thumb drive or USB HD, and perform clean installs for all updates and major releases. Far too much baggage is carried over from previous installs that causes nothing be problems. Apple needs to be better at releasing updates that don't break things, or at minimum, suggest doing clean installs (wipe the HD and start fresh).

I hope this helps others when they run into these issues. It really isn't super difficult to perform a clean install, it's just kind of a pain since we're all used to updates working pretty well for the most part.

Not the issue at all. When I enter my password, it just hangs with the spinning wheel and I never can get logged in. I know full well how to create accounts, so thanks for the pointers, just not needed.

Another issue is that other accounts that I had setup are apparently gone after upgrading to Catalina. I hadn't even noticed this but the root and admin accounts I had setup no longer show as an option when the login screen appears. Just my main user account. Again, entering the password here just freezes with the spinning wheel and I am never logged in. No access to the OS. I can ping it from another machine, but since I had not enable remote login, my access is denied. All the items I listed in my OP were from articles around fixing similar issues in either Catalina, High Sierra, or other releases.

Yeah, I started having all sorts of issues with Mojave shortly after I got my iMac with it preinstalled. I went with the migration assistant to migrate all of my files and such from an external drive, which seemed to be the wrong thing to do. Folder permissions went wonky after about a month of use, then come to find out that disk utility no longer has the function to fix permissions. Apparently this is done automatically, but that seems to not work right. Safari became useless around that time too. Tried all the different fixes that I could find/think of, to no avail. Other browsers worked fine, Safari just wouldn't load anything. Had to install Adobe Acrobat Pro for work, which was a complete disaster. The install froze a few times. Finally got it to install, then when activating, it froze the whole machine to where I had to boot in safe mode, and fix items that way (re-install of OS). Then when Catalina came out, it seemed like a great idea to dump Mojave. Initially it was pretty good, until the 10.15.3 update. Really bad experience as nothing seemed to fix the issue of not being able to login. Hence the drastic measure of starting over with a wiped internal drive. Like you, I really haven't been able to fully exercise the capabilities of my new iMac, which is a shame. Really dreading setting it up for DAW, which was my intent all along. I should be excited about it, but with all the issues I have encountered in its first 5 months of ownership, I just don't want to spend days/weeks messing with the OS to make everything work right.