I’ve been trying to solve this problem for a while. I’ve not yet found a really good solution, but I can summarize what I’ve learned, partly for your information but mostly in the hope that Cunningham’s law will finally put me out of my misery. Here are suggestions I’ve seen, organized roughly along some axis of easiest/most popular to hardest/least popular:

1. Get an NVIDIA Shield TV. This isn’t really what you asked for. It’s just a commercial smart TV box, but it’s generally considered the least annoying and highest quality of the lot. The unfortunate fact is that when dealing with DRM controlled media, having a big company like NVIDIA behind the product goes a long towards simplifying things.
2. Install Kodi. Kodi (formerly XBMC) is the elder statesman of the FOSS smart TV world. You can run it on just about any hardware, including a SBC like a Raspberry Pi. You can even get it pre-bundled with a Linux OS like LibreELEC. It’s got a clean interface and good community support, BUT it’s primarily oriented towards viewing media from your own collection. If you’re a person who consumes content via streaming services then you’re gonna have a rough time. Apps (mostly unofficial / community made) do exist for many popular services, but installing them can be a pain, and you may have trouble streaming in high quality (DRM issues).
3. KDE Plasma Bigscreen. Great concept, not maintained any more. See my comment here for all the gory details.
4. Clean build of Android TV. I’m not aware of any major independent android distributions (Lineage, Graphene) providing official builds of the android TV operating system, but this site seems to provide relatively consistent lineage OS based releases. You can run them on a Raspberry Pi. I haven’t done this yet, but it will probably be the next thing I try.
5. EarlGrey TV. This one is a deep cut. EarlGrey TV mad a very small splash in the FOSS news cycle a couple of months ago. The concept is simple: install your favorite Linux distro and configure it to boot directly into a browser displaying a static webpage with links to your favorite streaming services and/or local media folders. The implementation is extremely basic, but the upside is that it’s easy to tinker with if you’re so-inclined.

As for remotes, there are some decent options on Amazon that connect via bluetooth or a USB dongle and basically act like a mouse and/or keyboard packaged in a remote control form factor. I bought this one a while ago and it’s been fine. Nothing special, but fine. The play/pause/volume buttons on the front read on the receiving end like the media buttons on a keyboard. The air-mouse functionality isn’t for everyone, but this model is one of the few with a little track pad on the back if you prefer using that. Honestly just get anything with a full keyboard. So much easier than using the arrow keys to click-click-click your way through an onscreen keyboard.

I’m reminded of a quote from an old Tom Scott video. He’s visiting a modern reconstruction of a Neolithic long barrow. Tom points out that the sun lines up with the entrance on the summer solstice, then it cuts to the owner who says

I think a lot of people would assume that getting the alignments of a monument like this… would involve complex calculations, a sharp pencil, and computing power. But in fact, you can do it just as easily by getting up at the right time with some sticks.

I feel the same way about AI as I felt about the older generation of smartphone voice assistants. The error rate remains high enough that i would never trust it to do anything important without double checking its work. For most tasks, the effort that goes into checking and correcting the output is comparable to the effort I would have spent to just do it myself, so I just do it myself.