I was surprised that I didn't really like TI Flats on my BN5, as I typically love them. To my ears the LaBella LTFs sound the same on any bass I've tried them on, pillowy and indistinct, like a low-res version of the classic LaBella sound with less percussive punch.
I've used the old Sadowsky Black label flats everyone raves about (sounded great but the tapered B string was wayyyy brighter than the others).
I tried LaBella Olintos and pulled them off after a few seconds, they didn't really do justice to the versatile F-Bass tone and the B/E were super dead compared to A, D, G.
I'd personally go for DR Legends, or light Dunlop or Ernie Ball Group flats (but dunno if those come in 5 string sets).
All of those have a good balance of definition, brightness, and thump.