so now the main problem is building the hardware, there are a lot of solutions for the software part.
Before there were no general-purpose simulators, and barely usable computers (2 MHz computer with 2 KB of memory...), so all you could do was hardcoding the path and use rather constrained algorithms.
While cool and all, this type of sim is a tiny, tiny slice of the software stack, and not the most difficult by a long shot. For one, you need software to control the actual hardware, that runs on said hardware's specific CPU(s) stack AND in sim (making an off the shelf sim a lot less useful). Orbital/newtonian physics are not trivial to implement, but they are relatively simple compared to the software that handles integration with physical components, telemetry, command, alerting, path optimization, etc. etc. The phrase "reality has a surprising amount of detail" applies here - it takes a lot of software to model complex hardware correctly, and even more to control it safely.