3 to 4 weeks would be a great trip. Happy to offer some suggestions based on your interests. Check out a trip called the "Mighty Five".
https://www.visitutah.com/places-to-.../the-mighty-5/
I'd have second thoughts about visiting Zion just because of the crowds. But there are many scenic less visited places close by any of the 5 that I find very worth spending time in.
We'll be doing some off grid camping next week so responses may be delayed.