... a couple of hours later: My Gmail account is now 88% full with 4,228 unread emails and 13,932 emails in my inbox.
My method is to go backwards through my inbox. When I see an email from an account that is likely to have many emails, I search for all emails from that account. I did this to my messages from Duolingo and managed to delete over 550 emails, mostly unread, fairly quickly.
I have now got back to the pandemic. I was very active on Duolingo then - no surprises. I also had a lot of messages arising from people making changes to team Google Docs and Sheets. I was working as a magazine editor during the pandemic so everything was online. I was way too busy to do housekeeping on my Gmail account.
I have had the Gmail account since at least 2010 so I have got quite a way to go. I will leave it now, feeling pleased with my resolve and action today.