My personal opinion right now after testing many different slings: it doesn't matter, it's about what you are used to and for me what kind of sling you like making. For example I like making balearics and rockmans so I also like to use them.

I wonder if simnply making it thicker instead of stiffer would solve those problems as well. I have some relatively thick slings  and they feel very floppy and supple yet they do seem to have less twisting problems during rotation while still having a very easy and efficient turning-rotation (although at the cost of some increased air resistance.) it might be the golden middle way

I think there's more to twist resistance than stiffness (not saying you implied otherwise)
my slings lose stiffness over time, and vary depending on how humid and warm it is. I want to explore whether it's possible to do without real stiffness
we know stiffness resists rotation, and rotating is what a sling wants.
We also know that tension is very important for twist resistance
probably the type of braid and material too, and how that braid locks under tension
of course plied twisted cords under tension want to twist, so that can't be good,
although twisted  cords some benefits that can still make one opt for them, like almost no stretch at all and ease of construction.

So I am making a few more slings from cords I have laying around. 4-5-6 mm round braid synthetic cords but thesed
don't have a core it seems but they are very pliable, they lack a real spine. I also have other synthetic cords laying around
that are very stiff in comparison.

Thicker pliable cords... this is an option I haven't really considered all that much.  Keep the ease of rotation while still preventing tangles. Although the pouch orientation qualities I've yet to explore.
I think a lot of shepherd slings are this way,with their wool round braids.

Something different
I do think elasticity or stretchiness of a sling is not desirable, it makes it feel spongy instead of snappy

Okay so I did some more experiments with my thin slings today, and by increasing the gap between the gap between the retention and release cord, I was able to "whang" stones to much greater effect. I detract my statement about not being to whang light and thin slings, because you can, it just requires a very specific motion and potentially some adjustments to the grip.
Another thing that became patently obvious to me is just how important fast and 'perfect' spin stabilisation is. One might sling a projectile and see it fly straight and point first, but at long range it can lose enough of it's spin to become un-aerodynamic and drop (like a stone  ). This is particularly important if it has flattish faces, as they rob the rotational energy much faster.  I recon a projectile has enough spin if you see it corkscrew after release (due to magnus effect).
With the 28" 'Cortaillod" I managed 215+m (verified with google earth) using stones. Throwing along a calm river, it's easy to see the splashes up to the ~215m mark which was a log jam (and the best throws were traveling over it).
I will attempt to do the same thing with a Balearic of the same length...

I still don't like rotating the sling multiple times with the wrist prior to the powerstroke when I use a lot of cord seperation in the hand (which for me is a ring finger loop). It's not safe imo because it introduces oscillations up and down the cords which rock the stone back and forth in the pouch, while a tight grip does not cause this.  This rocking back and forth can (not saying it will) but can cause premature releases in my experience which are a danger to the surroundings.

To me if one wants to perfect the multiple rotations, it requires a tight grip, cords gripped together. This one can rotate freely as much as one like and there's no rocking back and forth of the stone in the pouch, there's no chance of premature release.
But as a consequence of the tight grip, the sling cords tend to be made thicker so they don't twist around while they are gripped and rotated together side by side in the hand, and so that one can still control the pouch angle with the tight grip. The thicker cords as a consequence add construction complexity and decreased efficiency. The tight grip already increases the spin rate, the thicker cords increase it even more. The spin becomes too great for the average coarse rock I believe. This I believe decreases efficency even more. All the more reasons for me to leave this sling philosophy behind and focus on the other.

For figure 8 or Greek style I will place the loop on the ring finger every time, because it doesn't use prior rotations that come solely from the wrist. Both styles are all one fluid movement, there are no prior rotations so there are no oscillations up and down the cords that can cause stone slippage caused by the wide grip. Thus all the reason to use the wide grip with these styles.

aside from that, when I use multiple rotations prior to the powerstroke, this is starting to feel tiresome to me, especially with a light sling and 200g or heavier stones, because it rotates so easily

Right now I definitely prefer figure 8 for everything. Because it fits thin slings so well. And thin slings are simpler slings I think, and more efficient. I get 300 g stones to fly point forward every time with figure 8 and with slings that use cords thinner  and floppier than 550 cord. Pouch orientation is not a problem at all because I feel this style mostly relies 95% on tension and the wide grip for pouch orientation, while styles that use multiple rotations prior to the shot require a certain level of sling stiffness to still be able to control the pouch I think. This I am starting to dis like because this adds complexity to the sling, but a complexity that is detrimental to efficency.  A simple sling is a floppy sling with thin cords I believe and this is an efficient sling. Pouch orientation issues can be solved through other means ( for example swapping to figure 8), instead of  making the sling stiff, thick and complex in construction just because one desires Balaeric style, although that is the trend on those Islands. And we rarely see figure 8 and thin slings there and they are convinced their way is the best way. its all compromises in the end and we have to simply seek a preference. I cannot say anymore one thing is better than the other but of course my way is the best way.  To each their own

Woah dude !!
Are you finding the projectile ??
How are you measuring the distance?
Any chance of filming the throws??

I wish I could get 200m confirmed, with the 27”

I’ve done 185ish lots and found the projectile.
All my possible over 200 are just guessing.