I think that’s true but South Parks humour has also changed over time. That’s the nature of satire - it lampoons human folly and vice, including the ideas of offence and moralising which are so often borne out of hippocracy.
You mention taboo topics like 9/11 as if it’s a no go area but actually that has been a rich source of comedy and satire due to the level of hippocracy displayed around it. The hippocracy of Uber patriotism, religious nationalism, racism (you mention people having to be careful about the target and culture of jokes, but many groups found the exact opposite after 9/11 - certain ethnic and religious groups were all tarred with the same brush, particularly in the US) and more. Even the idea of self censorship out of fear of causing offense. Some of this is being replayed right now with a contemporary conflict.
South Park is in a similar tradition to other satire such as Private Eye in the UK, or The Onion, or various other TV shows. South Park is just a sometimes more extreme version more willing to be deliberately offensive. But satire moves with the times like any other type of humour.
If you implement it from fresh then it is a new program. What matters is what your contract says about what you produce - some contracts pay claim to anything you make even outside of working hours.
Also if you rewrite it, while technically it is a fresh project if there are substantial similarities in how you implement it there could be an argument made that you have reused code that belongs to the company. Even if that is technical false it could be something you’d have to defend sometime in the future. As others have said, implementing the program in a different language and using a different methodology wherever possible should help protect against that.
I think the advice others have given that you should review your contract with a lawyer is sound even if this will be FOSS. It’s mainly about ensuring you don’t inadvertently open yourself to potential legal repercussions down the line, even if your employers at the moment seem benign. If you do work for a company that lays claim to everything you produce even in your off hours then I would strongly recommend you consider leaving or an exit plan, particularly if you are the sort of person who would be working on your own projects for fun or even your own business ventures.