What are the best practices of using a mailbox on multiple Outlook clients with Exchange Online

Question

In my company we're using Office365, with Exchange Online.

We have one mailbox, where multiple users work in it. Everyday between 5 and 20 users work in this mailbox simultaneously.

The users have Outlook 2016 with their own account set-up, and get this multiple-user-mailbox synchronized to their Outlook folder panel (on the left hand side) automatically since they all have full access permission on the mailbox (so it's in the same .pst file as their own mailaccount).

The mailbox itself is not a shared mailbox. it's like a user on it's own. it's set up with a Exchange Online Plan 1 license.

We have multiple problems with that, e.g:

• Sometimes the users don't see the same stuff. for example if user 1 changes a subject of an e-mail in the mailbox, user 2 still sees the old subject. this is a synchronisation issue probably. up to 15 minutes later, user 2 also sees the changed subject
• this mailbox receives all of the orders our customers send to us via e-mail. so there are quite a lot of e-mails. if we would delete for example 1000 e-mails from the deleted items folder, outlook would hang or crash on almost all workstation where the mailbox is set up
• sometimes, when a user drags and drops a single e-mail of the inbox to an archive folder, it's very likely that outlook will crash. this does not happen all the time, but quite often.

We tried to solve the problems by setting up the mailbox as a real shared mailbox but then we we're missing some features that we needed (i forgot them, but I know that we couldn't do it that way)

I don't think we're hitting any throttling limit at all. I don't think we're hitting the max. concurrent user setting of exchange online, and I also know that we're not hitting the peak of our internet bandwidth.

We can live with the first two problems I think, but I would really like to solve the third problem.

I would have asked this question like this how to fix outlook crashes when drag & drop an e-mail to a subfolder of a mailbox, but I think our Problem is a deeper problem, a problem in general. That's why my question is:

How can we get rid of our problems? Any guidance is appreciated!