Catalina reboot loop

I managed to salvage from the same situation by doing the following:

1) Connect wireless keyboard with a wire, to make it easy to...

2) Hold down Command-R during restart to enter recovery mode

3) Restore from time machine backup

If it crashes AFTER the restart, things will be a mess.

Here is the rough timetable of the Install Process:

Start download from Mac App Store. This takes 20 minutes to an hour.

Once complete, Installer usually starts. You specify disk on which to Install. [in Recovery, you may sometimes specify the drive FIRST.]

Install checks things out and then copies 5GB or more "MacOS Base System" to the destination drive. [In Recovery, "MacOS Base System" may be downloaded directly onto the specified drive.] Copy time is about 20 minutes to as much as 3/4 hour if your drive is slow. [if installing on the same drive as the download, this step becomes nearly trivial, and does not take any substantial amount of time.]

Installer sets startup to the new MacOS Base System and does a Restart.

>> a failure right here means you still have everything on your old drive, (plus about 6GB of MacOS Base system that does not work)

ONLY if Restart was successful do you begin replacing files to move to the new version of macOS. Time 30 minutes to 3/4 hour.

Restart again and comes up running the new version, in Setup Assistant. This is your best opportunity to bring over your Accounts and data from an other Mac or a backup, if you are changing drives or changing Macs. You can use Migration Assistant later, but it is more complex.