I use Kodi. I run it on older PC hardware running Linux, and on Raspberry Pis running LibreELEC. You can find refurbished Dell Optiplex Micro computers on eBay for under $100 that work great for this purpose. Small, decent i5 processor, quiet fans, they're pretty nice as set-top boxes.

I use cheap RF remotes to control the PCs, but one of the cool things about the Raspberry Pi is that the HDMI port has CEC enabled, which means lots of TVs are able to pass remote signals through the HDMI to control it, so you might be able to use the TV remote.

My main use for Kodi is as a frontend for my over-the-air DVR system based on MythTV, but Kodi has a plugin that will stream from your Plex server, too.