Medium Range Two Wire Bus Recommendations?
50 slaves sounds a lot like you want a proper network with flexible architecture instead of a shared bus, even if only to isolate problems. Your rather complex graph illustrates how little a daisy-chained bus would work for you.

As such, Ethernet is pretty popular, and mature. With IP and TCP atop of that, there's even proven software stacks that you can find for almost all microcontrollers that come with Ethernet interfaces.

These certainly are ubiquitous, and not too expensive. For example, the SAME53 family comes with a 10/100 Mb Ethernet MAC. Just add a RMII PHY (86 ct example), and be done: you get full networking, with collision detection, checksum offloading and whatnot. Since that thing is a popular ARM, you probably get to choose when (if, no need for IP in all cases) picking a network stack.

So: When building a network, like you clearly do, use a network, not a bus.