I'm not an heavy o365 user but i'm almost happy on Debian KDE with thunderbird 148[0] (email only), teams-for-linux[1] (chat/calendar/whatever), Onedrive[2] and webdav (sharepoint)[3]. Libreoffice/Onlyoffice for documents.
[3] Store the SP cookie via konqueror visiting the SP site, then open it in dolphin via "webdavs://CORP.sharepoint.com/sites/SITE/Shared Documents/" (sometimes the cookie is very short-lived)
[0] https://blog.thunderbird.net/2025/11/thunderbird-adds-native...
[1] https://github.com/IsmaelMartinez/teams-for-linux
[2] https://github.com/abraunegg/onedrive + https://github.com/bpozdena/OneDriveGUI
[3] Store the SP cookie via konqueror visiting the SP site, then open it in dolphin via "webdavs://CORP.sharepoint.com/sites/SITE/Shared Documents/" (sometimes the cookie is very short-lived)