Okay... and what about Intune? (Device management)

Entra? (User management and policy)

Office 365 Exchange?

Excel? (Finance runs on custom Excel macros and sheets)

Teams?

Office 365 in general, security, DLP, MFA?

>Intune

Fleet

>Entra? (User management and policy)

LDAP

>Office 365 Exchange?

Dovecot, Postfix

>Excel? (Finance runs on custom Excel macros and sheets)

Libreoffice calc, R and Python were needed. And if that doesn't work, finance needs to work around the vendor lockin

>Teams?

Matrix, Jitsi, Bigbluebutton, Mattermost

>Office 365 in general, security, DLP, MFA?

Authentik, Keycloak for MFA/security, OpenZFS with Nextcloud/Opencloud for DLP

It's possible, though of course less integrated and more work involved than just selling your soul to MS. But I am sure that time will also solve that, now that people are more interested in open source.