Well...
Don't know what to tell you, really. I've used DR Sunbeams, but it was a long time ago. And, they were short scale strings, too. I remember them as being... pretty coarse feeling. They were the strings that finally convinced me that my nerve-damaged, frost bitten fingers would be
much happier playing flats... They did sound nice though; the tension/stiffness? Average, for short scale strings, IMO... I have to say that they would be among my last choices for using on any of my fretless basses. And, NO, not for the "they'll ruin your fingerboard" thing. All of mine wear flats - or tapes, because they don't feel like I'm trying to play a wood rasp...
As for the La Bella LTF-4A low tension flats? I have those on my Custom Shop ('cause, left handed) G&L Kiloton lined fretless bass. And, I gotta tell ya; "super flimsy" is about the last thing they are. I've been playing basses with flats - almost exclusively - for 50+ years. I
LIKE stiff feeling, high tension flats - the stiffer, the better. I'm not sure how La Bella got the LTF's to have less actual, pull-to-pitch tension than TIJF's (I use those, too) but still
feel like "normal" flats, but... I'm very happy with them on my Kiloton. Just flexible enough to do what you'd like strings on a fretless bass to do. No, they are not as stiff as a lot of the flats I use on other basses; but... those basses are not fretless, either. My 3 other fretless basses wear Roto TruBass 88 Tapes; EB Type IV Flats; and D'Addario ECB82 Chromes. The medium scale Roto 88's are tied with the Chromes for stiffness, but... still plenty flexible for a fretless bass. The EB Type IV's (factory installed on my Schecter Stiletto 4FL) are...
slightly stiffer that the LTF-4As, but... nowhere near "super flimsy"... However;
It's your bass, and they're your fingers. Use whatever kind of strings you want - and be happy...