I like the Password for Nextcloud app. I self-host mine, but I think there might be Nextcloud instances that you can access. It is encrypted, and has an app for smartphones.

Do we know that this is the one and only domain that they will use when and if they actually enable federation ?

Something very simple on the web client: a setting to get links to open in a new tab, to make it easier to get back to the same point in the comments.

I am doing it because it is fun and it is a way to try to understand how things work. I am still not sure whether my answers successfully propagate to all servers.

Yes you need a domain but that is usually quite cheap