I use Plex from the Flatpak. It's kept up to date and works extremely well.
It plays mkv and will even, if the movie files are libraried (is that a word?) correctly, find subs for you.
It's a bit sensitive to file naming conventions and I use Tiny Media Manager to help with that.
https://www.tinymediamanager.org/
Edit: forgot to mention, Plex was available to install on my smart TV, so setup was negligible on that end.
Another Edit! - Plex is via Snap not Flatpak.