Here's my stone to the building.
Just like you, I like flatwounds that almost sound like roundwounds, but for other reasons.
I don't play with a plectrum, but I play fretless 99,99% of the time, so looking for the same overall feeling as you, but not for the same use.
I've tried almost everything available, yes it cost me quite a lot, but I always managed to resell the sets I didn't like.
I also have one question for you @dmc79 : Do you play in the higher register?
It's important, because a lot of flatwounds sound weird past the octave, out of tune in fact, even with a perfectly set-up intonation and it's more obvious on the E and A strings (linked to the construction)...
With that in mind, the best flatwound strings are the DR Legend, but they are a bit too dark for your liking, or the Dogal Hellborg Perfect Pitch Flat Wounds, but they are too floppy for your taste (and very expensive).
To me, the Fender 9050 are the perfect flatwound strings for your needs (and mine) and their intonation has improved over the years, certainly thanks to more control and precision during the manufacturing process at the factory.
Frankly, the Ernie Ball cobalts are as dull as possible.
The D'Addario Chromes are so stiff you could tow a truck with them, just like the Rotosound 77's and absolutely not the kind of tone you're after.
The Thomastik Infeld Jazz flatwounds are really good, very close to what you're asking, but the low tension is not a plus for you and sometimes the E sting may sound a bit dead to the ears, just like the LaBella Low Tension which are darker than the TI flatwounds.
The LaBella Deep Talkin' Bass will not suit you as they are indeed the sound of Motown as @fretmeister said.
You may also consider these Harley Benton flatwounds as they are close to the TI flatwounds sound wise, only stiffer and really cheap, worth a try even if the sanding process (to flatten the string) is not perfect, but won't be an issue with a fretted bass : https://www.thomann.co.uk/harley_benton_hqs_bass_45_100_flatwound.htm
Good luck with your quest.