Clearing Space in Google

20210419: This looks promising, and it is what I’ve done today.

20190129: This process ended up having many issues, opted for gyb instead.

Steps

1. Connect to account via Thunderbird IMAP
2. To help see progress, configure TB server delete settings to Just mark it as deleted
3. In TB, open All Mail, Quick Filter by Attachment, Sort by Date
4. For one year at a time, one message at a time, select message, and save all attachments to a folder, then delete (not detach, using Delete causes TB to create a ~250 byte (Gmail still reads as 1K) attachment with Deleted: [filename] as the filename in place of the original larger attachment. This file can be opened in a text editor to reveal more info about the original attachment. This creates a duplicate email in Gmail to the one that had the original attachments.
5. In Gmail, turn off conversation view so that the next step only removes specific messages with attachments, not entire threads.
6. Search for has:attachment size:20k before:2010/1/1 -Deleted to get a pretty close list of message threads that are the originals that had attachments. Delete or preTrash as you see fit.
7. Turn conversation view back on
8. Delete IMAP account from TB so you can have a fresh start next time.

ISSUES

Some labels may be lost. Message with replaced Attachments will not be in-thread as original was. Thunderbird doesn’t think messages with inline’d images are “attachments”, but Gmail does