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No need to delete one at a time, you can select a group and delete them all at once. The standard Mac selection techniques apply, of course, and there's an Edit->Select All command, too.

Deleting or archiving older items are the only options at the moment. The program will offer to archive periodically. There is no option that says "discard completed items automatically" but you can request one if you want with the Help->Send Feedback command.

As a practical matter, if the database size you reported is likely to be typical, you'll be able to accumulate a lot of completed items before you notice any change in performance.