Resolve sync issues between Gmail and Apple Mail

Around the 23rd of March my two Apple devices (iPhone and iPad) stopped syncing properly with my iMac (running Apple Mail as the email client). I’ve been fiddling since then, and as of this morning, when I deleted the Gmail account from Apple Mail so I could restore it to see if that worked it out, and now just about every mail Ive deleted or filed since 2019 is sitting in my inbox, with all of my filed emails since 23rd March now missing.
Desperate to get my missing emails back and to have all of the emails since 2019 go back to wherever I filed them/deleted them - are you able to diagnose and restore?