There is something similar to what you described operating right now: Stable Horde:
https://stablehorde.net/I'm not sure if it has the priority queueing system you mentioned, or how much it has generalized to non-SD loads, but it is functional.
Also, you should consider using a Huggingface diffusers UI/backend instead of the automatic1111 UI, which is based on (and now inextricably linked to) the old Stability AI implementation. Maybe thats fine now, but its already a problem for optimizations people are cooking up (like Facebook's AITemplate, torch.compile, SHARK/MLIR for AMD GPUs, Intel OpenVINO...) which work with diffusers already.
I am bouncing between InvokeAI and VoltaML at the moment (as I cant get any of the optimizations to work in the auto1111 implementation anymore), but I can hardly keep up with everything on Github.