It's a big mistake to ask advice here as everyone is different. I would go for steel fork, straight handle bars for a more rideable and comfortable position and wide puncture proof tyres which allow you to go anywhere you want.
Agree on steel fork, I've seen a lot of broken carbon frames/forks/wheels, apart from my own fork failure, not many catastrophic failures on steel parts.
Handlebars are personal preference - despite getting older I still like drops, wife likes swept back bars as used on "sit up and beg" roadsters. Neither of us like mountain-bike style bars where hands are at about 90 degrees to the bikes longitudinal axis, though we do have them on the Bromptons and one of my utility bikes (on this I use the bar ends a lot).
With an RST, the biggest tyres possible while still retaining reasonable mudguard clearance would be about 1.6". Marathon Supremes in 1.6" width are an easy-rolling but still comfortable tyre, with moderately good puncture resistance and still quite light. My wife has these in the 650B wheel size (in between 700C and 26") and thinks them a great improvement on the lightweight 700Cx28 tyres she had on her previous bike, much more comfortable without being appreciably slower.
I went for slightly smaller and more fragile 1.35" Kojaks on my RST. These are still quite comfortable, maybe a wee bit faster on good road surfaces but not as good as Marathon Supreme 1.6" on rough surfaces (towpaths, etc.). But I got my RST for fast riding on good roads, I have other bikes with 2.0" Marathon Supremes for heavy touring, mild off-road and general use.
