@zelphirkaltstahl I'm sorry this post will probably not help you, but I *think* I'm trying to build what you're looking for.
Unfortunately it's more like a neglected proof of concept (leisure project) than a robust product.

Here it is:
https://codeberg.org/aha/miniprat

These are the goals:
- Lightweight server
- Lightweight and static frontend (doesn't require js, if I remember correctly)
- Invite only (for small groups where someone in the group is server admin)
- Small and simple codebase
- Boring