Like roundwounds, flatwound strings differ a lot depending on manufacturer. There may be even more variation than between those roundwounds. There'll also be the differing subjective experience of the player and differences emanating from one bass or another.
Some examples: a Rotosound Jazz flatwound will never feel like a TI Jazz flat; the La Bella 1954 set is hardly like GHS Precision flats; Ernie Ball Cobalt flats really do not sound like Pyramid Gold flats.
But, I'd say, whichever flatwounds you decide to try, you'll always need to give them a little time to settle in, to find out how they will actually feel, how they will actually sound.
A couple of months should suffice. 😏