It is not uncommon to self-host everything except the outgoing sending. So you can mostly bring it all home without tackling sender reputation.
> Freshdesk. Can't exit because: we could get off the ticketing system, but the knowledge base is hosted here as well, and that's publicly accessible.
This can be done. The knowledge base sounds like some of the easier things to migrate tbh.
Why the need to go to paper filing? Airgapped servers are a middle ground.
But I guess your deeper issue is one of organizational culture norms, not of technical limitations or challenges...
Which I hope can be encouraging. It's all doable if you (plural) actually want it.
One path is to start with setting up contingency systems. Continously sync all mail to your own infra so you can access mailboxes even if o365 is unavailable. Mirror the knowledge base. Forward ticket mails to a duplicate archive (obviously potential caveats around PII and security here).