I believe the OP here is not playing 1999s music formats on a TV, but referring to MK4 and similar _video_ formats that a portable music player definitely can't play.
I believe in such places the cost of service and maintenance is many multiples the cost of the initial installation. A $20 "720p hdmi stick" from aliexpress vs a $300 mac mini is a no brainer. 100 mac minis will work reliably and predictably for years to come, whereas 100 random $20-$50 sticks sounds like something that needs constant supervision and maintenance.
That said, a RPi might be enough, depends on the software. That's why FrameOS has HDMI support, to work for such cases. It's still pretty far from being a turnkey solution today, but we could easily get there with some focus at the right time.
