A warning about Thunderbird: It sometimes deletes the entire email rather than just the attachment. It happens with both IMAP and with mail on your local disk. I'm on Linux with a vanilla install with no add-ons or extensions and I've seen this bug in recent versions and going back 5-7 years. There's at least one bug report filed and I think I also filed a bug report. Maybe it has something to do with my configuration because I would've thought many people would have seen it by now. I avoid deleting attachments unless I think the email itself is unimportant and don't care if it also gets deleted.

Yes, you can remove attachments with Thunderbird, but it requires a lot of configuration and it is highly error-prone. Disclaimer: I'm the author of Unattach.

I tried to use Thunderbird for this for a while, but there's a weird bug where it creates copies of emails that have the deleted attachments. You then have to delete those copies to fully delete everything.

Switch to the featured app here and everything worked seamlessly. Definitely worth it, as this way I don't have to pay for Google storage and can clean out a bunch of pure junk from my emails.