If you're up for using Jellyfin, you can check out something I'm working on to achieve this: https://github.com/MattMckenzy/Homehook
The main branch used to be compatible with chromecast devices, but I've stopped maintaining it in favor of the v2 branch, where HomeHook communicates directly with satellite devices through HomeCast. Working with Chromecasts were a huge pain.
Let me know if you have any questions about it, always glad to help!
For the multi-seating aspect, I've just set up something similar using something called a KVM Extender (I got this one). I have my PC set up in my office and another seat set up in my bedroom.
The KVM essentially just transports HDMI and USB a long distance through an ethernet cable, so the bedroom monitor just appears as a second monitor. I've set up a keyboard combination to switch between enabled monitors so only one is turned on at once.
Not sure how many seats you could push with this, since you would need a set of KVM extender boxes per seat with its own HDMI/Display and USB ports in the PC. Might be something to look into in case it fits!