I know I'm jumping in late to the discussion here. As someone who used a D80 for a significant amount of time and have upgraded since 2 times, I think I can understand your challenges.
You likely have been thinking higher resolution sensors with much higher ISO performance to be overrated. I can assure you they are not and this is critical to this style of photography. With natural light you need to step up the exposure by a lot. ISO noise is working against you big time. That is why your B&W images seem to work out better.
I would have to recommend a camera with higher resolution for starters. You might be surprised by how much more detail you will see. Think 16-24MP resolution and this is pretty much low to average pixel count now. The newer the DSLR body you choose, the better the ISO noise performance gets. My D80 images versus my D7000 versus my D750 just get progressively lower in noise and higher in detail. And I can work in much higher ISO normally. D80, I considered ISO 400 to be getting a bit noisy and gets much worse from there. On my D750 I will let auto-ISO be on and the image might record at ISO 3200. I hardly notice the noise.
Then there is the issue of the lens, and I'm sure you know better glass will only help more. I won't make specific recommendations as I believe any camera body made in the last 5-6 years (from any manufacturer really) will outperform that D70 from 15 years ago. Outperform it by a lot really.