In a lot of ways I agree with @bezo who questions if lofts are the best thing to use for this. TBH it is quite an unconventional use of a loft in the first place !
Ordinarily I would say it's not ideal, but I am aware there may be perfectly valid reasons why you might need it to be splines, or procedural etc etc...
But we can still apply polygon rules when using splines, and if we want certain areas curved and others very sharp, then we just need to make our splines do the right thing, which in your case would be this...
So straight bits get no intermediate points, curves get evenly distributed ones, and sharp corners get control points either side of the corner. But of course that is quite difficult (and perhaps impossible without recourse to nodal spline creation) to get procedurally. Which is a shame, because under an Extrude I'm fairly sure a manual spline like, with default interpolation that is giving you exactly the shape you need, and if you don't need to care about cap topology because it's flat, then that alone might be enough ?
CBR