I think that you could just rm -r tombstone and the directories will get created as needed when more tombstoned files get moved over. A slightly more paranoid solution would be something like
find tombstone -type f -exec rm \{\} \;
Still more paranoid would be to first rsync the tombstone stuff elsewhere for a while (just in case something got tomestoned by mistake).
The most robust (but scary nonetheless) solution would be to move images to S3 (AWS, or Digital Ocean). It looks like it’d be $5/month for 250GB on Digital Ocean. It’s probably a good long term solution and will reduce load on your server.