Hello all - so I too still haven't heard back from my Senior Advisor that I spoke to BUT I have done what the following link suggested (as @SometimesIAmRight mentioned above) and it appears to have worked:
https://buymymonkey.wordpress.com/2019/10/27/osx-catalina-cpu-spike-checking-mail-after-upgrade/
I've followed the steps in the 12/30 END OF YEAR UPDATE at the bottom of the page. I did not follow the keychain suggestion as I couldn't find any of the culprits that aspect listed.
It's been almost a week and after all my Exchange email downloaded again (over 450,000 emails) everything works fine, only Suggestd occasionally spikes but I think that's to be expected. But I regularly have been monitoring Activity Monitor and no Mail issues and been using the app fine now for days with no issues.
FYI on a side note, prior to the above solution, I'd been playing with Spark https://sparkmailapp.com . I don't like that you can't turn email threading/conversations off, but in the interim time I'd been experimenting with it I'd had zero memory issues. If they could turn the threading off, I might bail on Mail entirely. Oh - and other thing is that to save emails in folders you can only create folders in Exchange and save them there. Spark can't right now save emails to the local ON MY MAC locations like Mail can. Another potential drawback. But just throwing out there as a possible option. I get over 500 emails a day so I need something that works in a lot of cases and so far only Mail seems to be the best option but there's my two cents.
I still haven't heard back from the Senior Advisor I spoke to but at this point it seems I don't need any sort of response. Good luck all.