Find the core tone you want and work on things from there, I prefer the core tone of flats on fretless, but find TIs a little too flexible for FL, while for example La Bella Deep Talkin' Flats would be way to stiff (and I'm no fan of nylon strings for anything other than a very specific tone, one that I certainly wouldn't think of as versatile in any way). I do have a fretless strung with rounds and there are moments where I prefer it them, but if I had to make the choice it would be flats. Good luck 👍