If anything, this forum thread proves that we all have different needs. I too have looked for an Outlook alternative for Linux, and also have struggled. If I may, I would like to share what works for me, in the hopes that it will help someone else.
I still use Windows on my primary desktop for many reasons, however I changed my daily driver laptop to Zorin a few years ago, which has been fantastic (it also helped to squeeze a couple more years out of old hardware).
I am an IT Director for a couple of K-12 schools, and also have responsibilities at a university. Obviously I have quite a few accounts that span different platforms. The Linux tool that was the solution for me was Wavebox.
All Your Email Accounts and Web Apps in One Focused Place - Wavebox Browser
You can add multiple accounts (even multiple Office 365 accounts) to Wavebox. For example, for my work Office 365 account, I have added multiple apps that I can access at the top with one click (Outlook, OneNote, Teams, etc), and without signing in again. Each instance is separate, so there's no concern with one account affecting another.
Now, several posts in this thread mentioned that OWA isn't enough for their needs, and I agree that it is not designed to handle multiple accounts on its own. Wavebox uses the Web Apps for all accounts, including Office 365. However, since each instance is separate, all you have to do is click the icon for your account on the left in order to access OWA for that account. And yes, you will receive email notifications for all connected accounts. You can even setup a dashboard to show items from multiple accounts on one screen.
There is a trial available, and a limited free version. It didn't take me long to purchase the full version (which is a subscription for development reasons). The cost is not much, and it is well worth it!
Again, hopefully this helps someone. This solution works very well for me, and it has been very stable for the past few years.