How to properly downgrade from Catalina to Mojave with no internal free space left? Today I upgraded my Macbook Pro to Catalina, but after realising that I can't get files from my Samsung SSD and can't to few other features, I try to downgrade to Mojave again.

The downgrade method I read was through a Time Machine, but after an hour of recovering my stuff, there was an error window that says "An Error Occurred Restoring from Backup".

After that I tried to reinstall my macOS, but I can't do it anymore, because I have no space left on my Macintosh HD (only 300 MB free space, before all of this there were a 50 GB of free space).

What should I do? What is the best way to get back to my files? To delete everything on Macintosh HD via Disk Utility and then try to get back all of the stuff from Migration Assistant, or is there any better ways to do this?
Assuming your backup is good, use Disk Utility to wipe the disk from within Recovery mode, install Catalina clean (again from within Recovery) then use the Migration Assistant to restore everything. This should warn you if you're going to end up with a space problem and allow you to exclude certain items from the restore.