I am a fan of Flats...I have some on a P Bass and a Fender Jazzmaster.
SE doesn’t have flats for sale

I have seen a few mentions of Flats from users here.
If I wanted to drop some flats on 6,7 and maybe 8, what lengths do I need?

For what i know Stick strings use to be flats until mid 80´s

A little dampening gets you in the ballpark.
But it’s not the same.
I am experimenting with a few different materials.

You can determine this yourself by measuring your Stick and comparing to string manufacturers' 'winding lengths'.
Method: https://www.fretnation.com/Finding-the-correct-string-length-for-your-electric-bass_ep_46-1.html
List of winding lengths: https://www.fretnation.com/Bass-String-Lengths-by-Manufacturer_ep_47-1.html

For large gauge strings the tapping technique is helped by string flexibility. I expect the official Stick large gauge strings are custom made to be more flexible than typical bass guitar strings.
So it is helpful to choose a flatwound bass guitar string brand/product with high flexibility.

Thanks for the tech tips!
I’ve mostly played fairly normal basses/guitars and never learned how to read the math :

I actually just ordered a set of La Bella FLats for my Danelectro Bass VI
I’ll have to take a look at that and see if there’s anything there. THey are $50 so I don’t want to ruin them.

I’ll write SE and see if they have any advice