You could try the CALIBRE_OVERRIDE_DATABASE_PATH
Environment variable You'll have to move the metadata.db file too.
Untested.
Or you could do it with a symlink; i.e.
move the full-text-search.db file to your SSD, and create a symlink (named 'full-text-search.db') to it in your library folder. Tested on Windows - but it did not improve performance - still significantly slower than Windows and X1 with their search indices on a spinning rust drive.
BR