From their terms of service:

They (private repos) are also allowed for really small & personal stuff like your journal, config files, ideas or notes, but explicitly not as a personal cloud or media storage.

I’d guess that most private git repositories are small enough to fall under this category (unless you track large non-text files in git). This also seems like a very reasonable policy, considering that they’re a non-profit and they want to focus on supporting open source projects.

This might be what you’re looking for. When translating between languages other than English (German - Spanish in my case) the data can be a bit lacking but it generally works really well.

I’m using nebula to remotely access the raspberry pi in my home network and it mostly just works. The dual setup for nextcloud might be a bit more tricky, at least if you want to use HTTPS. You’ll probably have to set up a reverse proxy in Nginx for at least one of the routes, since they need different certificates (although since Nebula already authenticates and encrypts your traffic, HTTPS is probably not necessary there).

I think it's a fairly standard feature. At least Protonmail also supports this kind of "alias".