For starters, you'll never get a "perfect" finish on the underside of parts over support material. This is why 3D printing is such a black art for some people. Having said that, lets see if you can improve over what you have so far.
Rather than throwing values at you saying "Try that" , I'll try to explain why I suggest a change....that way you might learn what effect each setting has!
I'd change Support infill %age down to 15%, or even 10%...it'll save on material but still give support.
Increase the Extra Inflation Distance to 4mm. This will stop the sparcer support from tailing off at the edges.
Reduce Dense Support down to 1 layer....you don't need more than one really.
Change the dense supprt infill to 100%. This will give flat underside surfaces a chance to get a smooth support.
And finally, lower the upper separation layers down to 1. At 4 layers, it s a waste of time having supports at all.
This is just a suggestion as to a starting point.......once you know what settings do, you'll be able to adjust individual settings to fine tune things. One basic rule, though. Unless you know what you;re doing, never adjust two related settings at the same time...you'll never know which of the two settings made things worse..or better.