Cannot restore from my backup on PC

I foolishly 'updated' to BaseCamp 4.7.5, but found that it was cropping the location metadata on the JPEGs that it geotagged for me using 'geotag using track' function.

So I created a backup as I had done before, successfully before uninstalling 4.7.5 and installing 4.5.1, which I had been using quite happily before.

Now all I get after waiting for the restore function to work on 4.5.1, is a message 'Restore failed  The following problems occurred:  An exception occurred during the restore process.'

I've spent two days trying to get back my valuable data; walks, kayaking, cycling routes etc etc over decades and am panicking that all is lost!

I re-installed 4.7.5 and even 4.6 versions with the same error message.

I copied the associated files to a separate disc to 'start afresh' all to no avail.

BTW I have a lot of geotagged photos on my BaseCamp maps - 7,700, 32Gb plus; could this be a problem?

If all fails with restoring BaseCamp on my PC, can GPX tracks be extracted from the database folder...it just looks like a lot of filenames to me.  Some people on the forum talk about copying the database as quicker than using the backup/restore procedure, but can't see how to do that.

Any help most gratefully received!