Hello !
This seems super exiting, maybe a bit overwhelming to get into geometry nodes.
By each version doing something like that can become easier, but still GN is in it’s very infancy when it comes to working on stuff like that.
It’s kind of weird, because GN are done in a way that it’s probably possible to hack a solution but it can be very tricky.
A good example of that can be this experiment I did :
It’s inspired by a houdini breakdown. Original Houdini version seems much easier to make using loops, at least I get the feeling I could have managed something simpler with maybe blender 4.0 or 5.0 
…
All that said, with a little week of work I managed to find my own solution, and most of the time I tend to find a solution but it’s really not easy .
In the meantime, when all the tools are available for want you want to do right away, you can get super cool result really simply, it really depends on the case !
Instead of building everything in one giant nodetree, you can look for a semi-procedural system where you automate some part with GN but still model stuff manually.
Like, you manually place roads , buildings with simple shapes, and your GN tools will add details to that.
Python can also be a great addition, houdini relies a lot on scripting. What you’re doing with nodes you can also do it with a script and in some cases this can be much more convenient, it’s one of the things a bit lacking in GN if we wanted to do more advanced stuff.
But that’s only my opinion, you should definitely make your own and try to solve your problem in your own way !
BTW , there are a few city generator for blender, could be worth looking into how they do stuff !
Good luck and I’m looking forward to see this project evolve !