Gmail's new "Manage subscriptions" is handy but it only unsubscribes going forward. It doesn't touch the 500 old emails already in your inbox from that sender. MailScrub does both: unsubscribe and nuke the backlog in one shot.
It's a terminal UI that pulls down your Gmail headers into a local SQLite db, groups them by sender, and lets you blaze through them. For senders with a List-Unsubscribe header it handles the HTTP/mailto request automatically. For ones without it, it opens browser tabs.
Nothing leaves your machine — you auth against your own Google Cloud OAuth client.
https://github.com/brooksc/MailScrub
Biggest gotcha: you need to set up a personal Google Cloud project to get an OAuth client (~10 min, instructions in the README). I couldn't find a way around this for Gmail API access. Open to ideas. Only tested on Mac so far.