I've recently saw Cobalt Slinky Bass 5, but not sure what would be the best option using Ernie Ball Strings in order to play on a 5 String Fretless Bass...
I bought my first fretless a month ago, a used SR5. One of the first things I did with it was put on some flatwounds.
It's ... different. A very different feel than roundwounds. The strings seem thinner and they feel tenser, and heavier. In a strange way, they feel both softer and harder at the same time. I know, it doesn't make sense. But I like it. I don't know if I would like it on a fretted bass, but I find that they suit the fretless very well. And sliding is totally effortless.
As a bonus, flatwounds don't wear down the fingerboard as much as roundwounds. So ... I think you should give it a go.
By the way, I've also read that flatwounds don't have much top end. But in my experience, the difference isn't that great, and it's not something you cannot compensate for by using the EQ on the bass. Maybe they'll become duller when they get older, who knows. But for now, they're still sparkly enough for me. Even when I want to play (or rather mangle) some Primus. I just crank up the treble.