Update here, is working by now.
Main reason I'm not using "new look" is that it enforces routing all mails via Microsoft's servers. So for example, to transfer my mails out of outlook into a different archive, I'm using a locally installed IMAP server (in a docker container) which then is used by my archiving software (DevonThink Pro) to pull mails over to the local archive. See
https://github.com/mnott/mail2dt
Now that works very well in legacy mode. Why would I transfer mails that I already have locally over to Microsoft's servers to then have it down on my own IMAP?
So basically Microsoft formerly was breaking email only by using those ridiculous PST files you could never even really search in.
Now Microsoft is breaking email more fundamentally by interfering with your talking directly to whatever server you want to talk to.
Of course it's all their plan to lock you into their cloud.
Anyway.