"Add user to group" only works for the first group listed on My Account > Groups page
The javascript was simply not capable of handling more than one 'Add user' form. This will be fixed in 5.1.48 and 6.0.41
Even if you use mysql you still need sqlite3 for fulltext search (sqlitefts)
If you are running php-fpm as a systemd service, you could set UMask for the service.
The last parameter wins. In the above example the object with id=2 is selected.
It's on my agenda for a long time. So, I guess it's about time to do it.
If you need to migrate all data from one seeddms installation to another then just copy the data folder and dump/import the database.
Do you really need this tools? Which database are you using? Are you planing to change it?