you could use the Resample (Spreads) node to resample your input into nice curves - i don’t remember exactly if it can do B-Splines, but other options might work equally well (your input density appears to be pretty good on the screenshots).
that would let you interpolate the curves independently, by making use of the Bin Size Pin.
thanks diki. It’s too bad Resample doesn’t just implement B-Spline. I don’t think the interpolation methods in Resample would look as good, but I’ll give it a go anyway.