Do it in batches by sender. Do a search by the sender and then select all. Delete. Find a new sender, rinse, repeat. Keep at it until you get down to a manageable level which you then handle by subject, rinse and repeat there.
Ive found couple of messages that were different sender names somehow interjected...in a long “list” of messages by the “same” sender and this has slowed the process down somewhat
example
abcauto
abcauto
abcauto
B+H
abcauto
abcauto
had i grabbed everything Id initially selected for deletion..I would have deleted one message I wanted to save.
I needed this lesson in housekeeping I guess.