> From what I understand, Collabora is LibreOffice in your browser

Indeed:

https://www.libreoffice.org/download/libreoffice-online/

Does this include hosted email? https://www.collaboraoffice.com/collabora-online-3/ shows that it has a user directory, but I can't find any direct mention of a web email interface.

https://www.open-xchange.com/ does email, office, and solves many enterprise management problems already. It can be cloud hosted or deployed online and is mostly open source (https://gitlab.open-xchange.com/frontend, pretty sure they've got everything but the "pro" stuff licensed under GPL). It's based on OpenStack, though, so hobby deployments may pose a challenge.

It's the most complete European office product I've seen so far.