Maybe it's obvious, but how about Kodi?
It has support for CEC (for utilizing your TV's remote through HDMI), of course there are other input options as well.
There's plenty of plugins to support online video services. Needs some work to configure them, and sometimes it requires maintenance when a plugin stops working, but I was in general happy with the way it worked while I used it for a few years.
It's worth mentioning that HDMI CEC is spotty on Kodi. I've had no problem with it on a Pi or CCwGTV, but if you try to convert a desktop or laptop into a media center PC, chances are the HDMI output won't do CEC, and you'll need an adapter (further reading: https://kodi.wiki/view/CEC#CEC-capable_HTPCs)
Nothing in Plasma Bigscreen is specific to ARM. You just got to find a distribution that ships it for x86 or compile it yourself.
But tbf I wouldn't recommend Plasma Bigscreen, it currently has no developers and is missing a lot of polishing and general apps.
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.
As far I know, for Plasma Bigscreen you can use KDE Neon as well, which you can install on x86.
Arch and alpine linux have packages for plasma bigscreen...
Not specifically designed for that, but I'm happy with Sunshine + Moonlight ran with controller.
Take a look at Jellyfin
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