Whew !!! well, Macittaja.... you can go back to a previous version of the MacOS, but it's not easy...
Step one: Have you done/did you do, are doing now. and still do a backup and it's good (verified solid backup....)
according to the pictures you've posted (thanks for that, it makes my (and our) job easier and explains/clarifies pretty well. So....Do you have, or can you make a Mac formatted USB bootable stick/thumb drive/flash drive with Mojave (mac os 10.14 on it..... you'd need to download Mojave this way: How to download and install macOS - Apple Support (CA)
plus you'd need a USB stick/thumb drive/flash drive with at least 8 gigs capacity, but 16 gigs would be preferred, especially for Mojave....and you'd need to Mac format the USB stick/thumb drive/flash drive, after you plugged it into your Mac..MacOS Journaled, extended, GUID on and Boot drive..... after you have downloaded the Mojave installer, you'd also need a Mac bootable USB stick/flash drive/thumb drive maker/application/program....I used LionDiskMaker successfully in the past, but I'm sure there are others. There is also a way to make a Mac USB bootable thumb drive/stick/flash drive using only the Terminal, but I can't for the life of me remember what it is....After you have downloaded the mojave installer app,and unzipped it, it should be in either a .dmg or .img or .zip format.... then run the Mac bootable usb stick/flash drive/thumb drive maker, ensuring your USB stick/flash drive/thumb drive is plugged into your Mac...and for the source, point it to the Mojave installer app..... let it do its thing.....when it says "done, finished, ready", then safely eject the Mac formatted USB bootable stick/flash drive/thumb drive.... you're going to need to, I think, start from scratch....and you have a backup (YES !!) so you may need to erase the Internal drive.... don't worry if it looks funny or is divided into two parts, from Catalina onward, it splits it into two parts, one of which is protected..... you could run Disk First aid from Disk Utility first....and once it's verified that your SSD *(I think that's the internal drive) is totally good, fine, no problems at all, then you could erase it......then do a reset smc/pmu followed by 3-4 pram zaps on startup, just to be on the safe side, following this link...https://www.macworld.com/article/224955/how-to-reset-a-macs-nvram-pram-and-smc.html....... then plug in your USB stick/flash drive/thumb drive, the bootable one, into your Mac and immediately hold down the "Option" key on your keyboard, this will get you to startup manager, where you can pick which drive you want to start up from, which includes USB drives and the CD/DVD drive, plus any internal or external drives...they all are showing up there....use left and right arrows, or mouse, to pick one, then mouse click on it, or Enter on your keyboard to Confirm, say "Yes", OK/ , that's the one you want, you should pick the USB stick.... it should open up and get going and allow you to re-install Mojave..... when it's done, don't forget to make the Internal drive the start up/boot-up drive, rather than the USB stick/flash drive/thumb drive.... say "OK" to that, then restart your Mac.... when your mac has restarted/rebooted, and Mojave has finished loading and has got going, run Migration Assistant, or go to disk Utility and then Restore.... as for source/where from, where it is, point it to your Time Machine Backup.....and let it run.....and be patient, have faith, cross your fingers, and everything will come back, hopefully
Hope this sorta helps you
John B
