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)
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.
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.
Arch and alpine linux have packages for plasma bigscreen...
As far I know, for Plasma Bigscreen you can use KDE Neon as well, which you can install on x86.
Not specifically designed for that, but I'm happy with Sunshine + Moonlight ran with controller.
Take a look at Jellyfin
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
Community icon by Alpár-Etele Méder, licensed under CC BY 3.0