MS 365 Outlook Desktop
 

When doing a "repair" on 365 recently MS helpfully decided to remove all my mailboxes and switch to "new outlook" app. It doesn't seem to be possible to revert to "classic outlook" unfortunately. There also seems to be a mailbox limit of 10 accounts including non MS accounts and the registry edit that used to increase this limit no longer seems to work.

MS forums don't seem to indicate a solution to the account limit cap. If anyone knows of a solution pls it would be good to know.

Are there other suitable options for desktop email apps which support multiple mailboxes/accounts worth giving a try out?

Have you tried Mozilla Thunderbird? It's been a stable email client for decades at this point.