Factorio

using train limits and circuits to calculate the limits

This makes me happy. This is the way :)

Yes, 8 wagon trains are quite big. 1-4 goes a long way. 2.7k is well doable with 1-4.

I also want a single fueling (maybe adding multiple as the factory grows) station that is visited by all trains.

So each train has a schedule of 3 stops? That means they will visit the fuel station much more often than necessary, which creates more traffic than necessary. When you get to 50 trains or more, traffic and jams can start to cause issues which are hard to solve. You could reduce the issue by making longer schedules (load, unload, load, unload, ..., refuel).

Another option would be to incorporate the refueling into large stations. For example, ore trains can be refueled at the smelter unload. You can have one dedicated fuel train delivering fuel, and the ore trains can have a simple schedule (load, unload).

I’m trying to be a bit more organized in the creation of this one

If you enjoy making blueprints and planning ahead, make a rail book. Stations, straight, turn, T- and X-junctions. Make sure they align when rotated. Grid aligned can make sense, but isn't necesssary.

Or grab a book from the internet.

It also helps to move smelting / production sites to rail based outposts as soon as possible. This way, your main base won't need to become messier and messier as you progress.