Converting personal ICloud into generic accounts.

I was reading before, but sadly, i couldnt find any answer - maybe you guys can help.

Historically the certain department of the company started using personal icloud accounts as generic - meaning there are 15 people working with the same icloud account as a mailbox - attending emails, logging in and out. This leads to the flags disscinchronisation that is impossible to track down (e.g. someone puts a flag, someone removes), might also be a problem that MacOS in the department is different versions from Mojave to Big Sur, also it leads to security leaks - as users who have access to the mailbox can track down the devices linked, see the addresses, contacts, even tabs of one another.

Basically we strongly need to convert these icloud accounts into shared mailboxes, generic accounts that won´t be private. Or maybe even migrate them to our own mail server. Is it possible? Or we need to handle a long and unpleasent procedure of exporting the data and importing it into mailboxes of our servers? Also there are about 150 rules on one of the mailboxes - i imagine these won´t get saved once the data is exported.