Found it. Go to Accounts and Passwords > Gmail > Accounts > Advanced then change move discarded messages to deleted mailbox.Yeah, no go. Even within settings my swipe left and right options, I've got: none / mark as read / flag / move message.
In mail, inside gmail inbox, when I swipe: move / more / flag / archive. Outlook outbox: same gmail options, but trash instead of archive. This is pretty annoying. Might just end up installing gmail.
"It just works."