>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.