Haven't tried it recently, but last year, I created a new separate account on my MBP and moved all of my work and mail over to it. Mail was a pain to move, but it seemed to solve a lot of problems at the time. Having too many email accounts in Mail seems to be a part of the problem, especially if they were Office365 emails. Disabling some of the emails in Preferences calmed things down, but I need them for work, so not sure what to do. Funneling several email accounts into one helped, but did not fix the problem.