Well, if you played upright, now I understand...then fretless electric would be no problemo..for you.   But I only have played electric bass...so fretless is more than I can handle.   I'd love to/plan to get into acoustic upright Bass..just a matter or when. 

Speaking of slide...ever tried "Slide Bass"?   I did a tune once on electric bass with a slide..very interesting sound possibilities.  Not original though...I think there have been other slide bassists.

I have heard that flat-wound does spare the neck on a fretless.  Don't know if the tone vs round wound makes a difference.  Apparently Jaco had a preference for one or the other, I don't remember which.  He also lacquered his fretboard with something that made it super smooth and easy on the fingers, although the original purpose of the lacquer was to fill in the gaps where he pulled out the frets. 

G