Linux

For software, Kodi is very good and probably easy enough for your SO to operate.

If you want a very small, low-power device, consider a Raspberry Pi 5 (or maybe a RPi 4 if you don't need high-end A/V output). Raspberry Pis are tiny, and unlike most other single-board computers, their video processor correctly handles fractional frame rates that are common in broadcast video. LibreELEC is a ready-made operating system for these devices, centered around Kodi.

If you want more versatile (and more expensive) hardware, NUC-class mini-PCs are often praised for use as media centers.

If you want to build your own system, consider an AMD Zen 5 or Zen 4 CPU, or maybe Zen 3. Most of these have an integrated RNDA2 GPU. That iGPU only has a couple of cores, so you won't be doing much 3D gaming without adding a graphics card, but they are more than enough for hardware-accelerated video decoding/encoding.

This is the air mouse remote I recommend with a dedicated 2.4 GHz connection. Also, recommend Plasma Bigscreen QT6 version.

If you don't want to rely on Bluetooth, there are lots of keyboards, mice, and media remotes with dedicated dongles.

I use one of these, for example: https://www.amazon.com/gp/aw/d/B07ZKL66GY

I have a Pi3 B+ in the minivan for the rear seat entertainment system. Runs OSMC, uses a 128GB flash drive for media storage, and for the remote, I programmed a Flirc IR receiver to a spare Roku remote. OSMC is also easy to lock down so nothing gets deleted nor settings changed by accident.

This is the way.

I use I think this one: http://www.riitek.com/product/254.html

It has orange buttons that you program to the TV (I have on/off, volume, and changing input source) then the white buttons and the direction pad to move around and play things in Kodi.

Get a mini PC with an intel n100 and get a flirc remote adapter. Low power, quiet and plays 4k content perfectly in Kodi

My setup is a pi4 with OSMC that is just Kodi all setup, plugged into hdmi with CEC to allow remote pass through.

That pi has an nfs mount to my jellyfin server and the movies and TV shows indexed off the NFS drive.

It's setup is so simple even our now 6 year old can put a movie on

kde connect has a really good media controller/wii joystick mouse thing for your phone

I don't have much issues with Bluetooth in Linux, works just fine on my Kodi installs, audio, remotes the whole kit. But I can recommends the G60S Pro remote. It has both 2.4Ghz and Bluetooth so you don't have to rely on the Bluetooth if you don't want to. Flip it and it has a keyboard, incredibly useful!

Before that I ran CEC remotes (found is kind of meh), IR receiver (worked fine enough), and other 2.4Ghz remotes (MX3 variants, including a Bluetooth one).

I also use the companion phone app, Kore, on a Android.

Like others here I've been running Kodi for nearly 20 years, has amazing spousal approval rate. I just redid my setup, PXE boot a couple of Pis and an x86_64 machine (and Qemu for testing) it's a bit of a pain to share more than the libraries (my MySQL), but that's really more than most people want anyway (and I did manage to share almost everything with some NFS trickery, advance config path substitution, bind mounts, etc).

I've been using a linux box auto-starting Kodi which is controlled by an MCE remote for about 10-15 years now. Both my partner (who is tech negative, seriously everyday they yell at their work laptop) and 5 year old kid at the time had no trouble with it. I personally love it. With the add-ons and such I've basically turned it in to a Home Entertainment Center. I've download Lutris and with a little tweaking I'm emulating a ton of old games and I've installed the Kodi add-on that lets me integrate Lutris to I don't have to leave the Kodi environment and bale to desktop is which is great. I've also got an add on in Kodi to open Steam in big picture straight from a button on my Kodi favorites. So I can go from playing Slay the Spire on Steam to watch an episode of Scrubs to playing Mario Kart Double Dash in Dolphin, all with out seeing the desktop.
On the other hand I haven't invested in one of those fancy remotes that lets me power on the PC so before I sit down I got turn on the PC which means the PC sits on a shelf in the living room. But to be honest we often leave it on for days on end, which I suspect racks up the power bills a bit, but lets me just turn on the TV and go. I also haven't invested in a HDMI - CEC set up so I have to have two remotes one for powering on the TV and a second for everything else. And getting Linux to recognize my 3.1 soundbar is a forking nightmare it still only see it as a stereo soundbar but hey it still sound good.
Over all I really love the Kodi HTPC set up for the heft of added home gaming stuff,but if you're looking for something more low profile I have a Raspberry PI running OpenElec stuck to the back of my bedroom TV with double-sided tape. OpenElec is a branch of the Kodi main steam for small PCs. The bonus on this is that the Pi's have built in CEC so you will only need the one remote.

I put Moode in a Raspberry Pi 4 and control it with my phone.

Think about it, what better remote do you want? You get album covers, lyrics, the works. You can even stream from your phone to the Pi.

My needs are more centred around gaming but I built a SFF PC based around the 7800X3D, 9070XT and FormD T1 case (sub 10L) which sits under and connects to my TV and hifi system.

I'm therefore just using the TV as a monitor, and can access my NAS through the PC to play media directly from it, as well as game through Steam ofc. I've not had issues on BT with EndeavourOS but am looking forward to switching to the Steam Controller with it's reduced latency with multiple devices

The bare minimum viable is an AMD CPU of some description, and an AMD GPU that can at the very least decode h.264 and h.265; an FX-4300 or R3 4100+RX 550 combo would be a good example for the bare minimum viable Linux HTPC build, as Polaris cards like the RX 550 can decode h.264 and h.265.

Ideally, you'd have an AMD CPU and an AMD GPU or integrated graphics in the case of an APU, that in addition to decoding h.264 and h.265, can also at least decode VP9 if not also AV1; an R5 3400G would be a decent HTPC APU in that case since Vega integrated can decode VP9 in addition to h.264 and h.265.

Beyond the hardware, set up the appropriate hardware acceleration so you can decode video through VA-API, and then set up Kodi and get a cheap tablet to install Kore on and use that for a remote.

I personally prefer Android TV for its app support.

My main concern is replacing the remote

well, if you're using something like Kodi you can just use a controller. but you can also get handheld trackball mice that are held like a remote, that work over the usual 2.4GHz.