No company is just going to host a server for you for free, a (virtual) server for running nextcloud will cost you at least a few bucks a month. As others have already said, you can run a server at home on your own hardware, but this is also not free (hardware cost, electricity, etc.) and you will additionally have to deal with any hardware issues & replacements yourself.
I think it’s a fairly standard feature. At least Protonmail also supports this kind of “alias”.
I think Plex is not open source but is there any reason against Jellyfin? As far as I know, they also have apps for most smart TVs in case that’s what you mean by navigating with a remote.
In my experience, a good way to get a polished desktop with a tiling workflow is to use KDE / GNOME with a few extensions & i3 shortcuts. Unless you really care about customizing every part of your DE, the work of configuring i3 to match a proper DE in terms of polish might not be worth it.
I previously used GNOME with the Forge extension and a few simple extensions for a workspace indicator, disabling the workspace switch animation, etc. This worked quite well but since GNOME is not very configurable, you have to do a lot of that through extensions (e.g. disabling the workspace switch animation & popup). This is particularly annoying since GNOME updates tend to break extensions.
For that reason, I recently switched to KDE. Polonium is a very nice tiling plugin for it. Since KDE is pretty customizable, I didn’t really need a lot of other extensions to support my workflow. It’s mainly a matter of configuring keyboard shortcuts and a few other settings. I haven’t used KDE long enough to say how stable everything is under updates, but from what I’ve heard it should be a lot better than GNOME.
I personally use NixOS and Home Manager with the Plasma Manager module for KDE. It’s a steep learning curve but if you have fun learning new stuff it is worth it in my opinion.
Otherwise, a GNOME / KDE tiling setup will probably also be mostly reproducible if you just track your dot files. There’s always a bit of manual configuration but it’s also difficult to completely avoid that with NixOS (although probably possible).
That can be a good solution at least if electricity costs are not a big deal. If power is expensive in your area, it might be worth to buy something more power-efficient, like a raspberry pi (assuming they’re not completely sold out right now).