OK go to System preferences, Users and groups and create a new user with full Admin privileges.
Log out of your normal user and log into the new user account.
Set it up with the same AppleID
When set up, set up Mail with your AppleID mail
Test again
If it works OK then it proves a corruption in your old user ~/Library
Bad news is it is a devil to fix in your normal User account. There is no "Reset Mail Settings" command. You need to first close Mail and then delete the ~/Library/Mail/V7 folder
Then with the app Easyfind locate and delete every file in ~/Library with "Mail" in it. Even then you may not have fixed it.
Easier to move all your user files to your new user account.
Anyhow I am ahead of events, try the new user account!
PS to find your ~/Library in Finder click Go, then Go To Folder, then copy and paste
~/Library
into the box and hit enter