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).
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 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).
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).