I personally use Thunderbird - IMAP and CalDAV - and have no issues although I am using an older version as I don’t like the 102+ versions (yet). I’ve not seen the memory issues you describe in v91 and below. I’ve just been monitoring it with several accounts active and configured and can’t get it above 1GB of memory usage
The trick has always been to wean people off of Outlook! I’ve had success doing so with EmClient. It is nice, but I haven’t used it as the ‘daily drive’ for some time.
You could look at BlueMail for Windows if ActiveSync is important.
The other solution I quite like is ‘Appify’ Sogo webmail. Use (eg) MS Edge to log into the webmail; select the dropdown menu top right; select Apps; Install this site as an App.
Pin that to the taskbar. Works a charm once you get used to getting around SOGo webmail (which can be counter-intuitive at times).
Klaus