I'm giving MailMate a try again after using Mail.app for the last six months. I accidentally deleted my custom keybindings 😢 so I had to re-create them. Here's what I came up with, on top of the built-in Gmail keybindings:
Note that some of these are not listed in the official custom keybindings documentation.
So far it's good to be back in MailMate. Mail.app has been getting slower and slower, especially search, which prompted my switch. So far I like the (fairly minor) changes to MailMate since I used it last.