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.
It's crazy how much money is wasted in the US, where other people are begging for food on the streets.