Interesting. I'd think if a lighter beer was thin, a darker beer would be as well. Maybe I don't understand what you mean by thin. A lack of body?
If it's a lack of hops, you could consider a sulfate addition. CaSO4 for example.
If you move to RO, you'd still have to add a bunch of stuff to it, perhaps even more than now. It's a clean slate to start with, but also just that, a clean slate with nothing to contribute, good or bad.
I'm struggling to find one, but I'd think there *must* be a table somewhere as a starting point, for various styles. You don't have to stick with it but it might be a place to begin. A pale ale and a chocolate stout will have different suggestions for example. Or an English ESB or a pilsner.