When you're trying to clean up the trash on the web client, you can either select every message one-by-one or select all of them at once; you cannot select, for example, the oldest 500. The selection checkbox only has options to "select all", select read/unread and starred/unstarred.

I currently have 8000 messages in my trash (I just found out that it doesn't auto-delete on free accounts 🤡), so I'd like to delete only the oldest 7500. For safety, I don't want to wipe all the newer messages; who knows, I might need to recover something in there.