If anyone is interested.
Since I have the same problem (BasicAuth disabled in my tennant, osTicket 1.10), I wrote a litte O365 pipe-Gateway that will read the incoming emails via GraphAPI (App-Credential!) and pipe it into osTicket remotely (api/tickets.email). Works like a charme.
Currently in C# , but should be adoptable to any lang without real problems.
I'll publish it on my Github the next days, after production tests