changing the background pic

if you are the sole user (not even a guest account) AND file vault is turned off you can go to System Preferences > Users & Groups in your Login Options (unlock the padlock if necessary) and set "Display login window as" to "List of users". then your login screen matches your desktop background.

if you are using file vault or have multiple users, the above will not work.  if that's your situation, you will need to wait for apple to address this. it may be worth your time to send feedback directly to apple. Feedback - macOS - Apple. 

Simplicity, I have more control, plus 10s of $1,000s of perfectly good software, which is either too expensive & gone to subscription mode, or not available for updated OSes.:)

I can & do run all the latest OSes to keep up, but they provide no benefits for me.

Here ya go, from one of my favorite Developers...

https://nektony.com/how-to/change-lock-screen-on-mac

Not that I'm aware of, but I prefer to run the oldest OS version I can get away with.