Hopefully this is the right kind of question for this community, my apologies if not.
I’m looking for some sort of device that will let me load my own media from a USB / SD card, and play it to my TV. Does such a device exist? Some sort of remote controlled box that can just load mp4 / mkv files from locally hosted storage? I have been using my PC over HDMI but it can get annoying having an awkward room layout because my computer always has to be next to the TV to do this.
It can be a bit overkill for your use case if you only need to stream the USB media on your tv, but take a look at Jellyfin, it’s a program you can install on any PC and as long as this is up and running on the same network you can access your media on that PC (in your case with the USB plugged in) from any other device (TV, other PCs, Tablets, smartphones)
Like kylian said: Kodi. You can install it even on an android device (like a firestick) or you can go for some distro giving an out of the box good experience like LibreELEC.
I suggest to avoid the temptation to get one of the many cheap Android boxen meant for media playback from Ali Express or the like, as they have a strong tendency to be heavily loaded with malware. Definitely not all of them, but it’s really hard to tell which specific one you’ll get.
a small PC with kodi can do this. It can do much more as well.
There are a lot of tiny PCs these days that can output 4k video and audio. Look for something with an N100 or N200 CPU if you want to go as cheap as possible, they tend to be super-cheap and perform well. I’ve got one of the GMTecs and this wireless keyboard+mouse, works really well from the couch.
There are cheaper/other options but to get you started: https://www.amazon.com/GMKtec-Windows-Computer-Business-G3-dp-B0CQ4XQ2WG/dp/B0CQ4XQ2WG https://morefine.com/collections/pc-box (specifically the M9)
My solution is a bit old school: A raspberry pi connected to my network and running miniDLNA. It has an externally powered USB hard drive. My TV runs Android and I have VLC installed. Any DLNA client works including Xbox and mobile phone apps too.
I don’t think mini DLNA is even updated anymore so eventually my solution might stop working but it’s been running solid for 10 years