I havent looked at geometry charts, but the seatpost looks a bit like a triathlon seatpost with the ability to move the saddle mount fore / aft. But if that's the case, by how much?

I haven't actually measured the exact amount of movement possible, but I guess 15mm in total.

What size is your bike and how high is the saddle if I may ask?

Size 56 and saddle height is 78,5cm.

Are these much stiffer as in harder to bend and harder to route around tight turns?
I'd like to make the rear brake on my rim brake bike less spongy. But since the cable is completely internally routed, compressionless housing is not really an option.

I have combined compressionless with regular housing in the tight turns.

You can shop around, but I believe this store is the OEM that makes them. Just make sure to order the road version and get the right length of cables for the front and rear:
https://www.aliexpress.us/item/3256804589342357.html

When I installed them, I noticed a bit more tension/spring back on my Juin Tech F1 brakes. Also visually they look a bit more premium! Lol

I attached a photo of how they look on my bike.

Thanks, but unfortunately the link doesn't work for me, I just get "page couldn't be found".  The cable looks very good!

Did you end up doing anything to your fork steerer or was it fine after you cut it down?

Since I had to sand it down at two places quite a bit to make the cockpit-stem fit, I gave it to a carbon repair guy who glued in an alloy insert that goes down far below the headset bearing.

The communication was always friendly and I did get a partial refund. But the seller did not really seem to acknowledge the problem.

This exactly matches my experiences with the seller. I too wouldn't buy there again. Would be interesting to know how TanTan handles cases of obvious deficiencies like my very much unround steerer tube.

Short follow-up on my steerer tube issue: I opened a dispute on Aliexpress and after 3 weeks of sending photos, documents about acceptable tolerances etc. I got a refund of 100 Euro + 20 Euro because the seller sent me a cockpit of the wrong size (400x100 instead of 420x90). Now I'm going to cut the steerer, sand down the areas which are out of specs the most and then send the fork to a carbon repair guy who will produce and glue in an alloy insert to strengthen the steerer.
Btw this is my frame (chameleon blue/purple & matt black):

Oh wow, this is pretty brutal - did you open a dispute?

Atm I'm still in a discussion with the seller and I'm in contact with a carbon repair expert to see what options there might be...