Make an in-wall cable duct yourself?

Can we install it ourselves afterwards?

What exactly is already finished? You are talking about the developer; usually, you get the house ready to move in, possibly minus the floors. That is different from working on the shell yourself.
You can ignore structural issues. Correctly, you slot vertically, so it only affects a very small strip.

Unfortunately, the developer did not offer an in-wall cable duct.

From your thread history, I deduce that you actually mean a general contractor. The apparent consequence of interpreting "building with GC" as "building without your own delivery and service specification" is as silly as it is popular, and it shows in such details. I find the suggestion from with the drywall to be the "logical" way – authentic, because this is just the GC’s style, to fix missed detailed planning with drywall. Another way would be drilling through the wall and milling the cable duct into the insulation layer. However, the drywall is the technically cleaner alternative and can also be used immediately for wall articulation: for example, by building a kind of mantel around the TV and/or the sides of a wall-integrated shelf from it.

The biggest concerns we have are regarding the static structure or whether a thermal bridge will be created.

Do we not have to worry about that?

I found a channel that is 6cm deep
Outer diameter 5cm

and it is 8cm wide, all cables should fit there.

Yes, the cable duct would of course be vertically slit, inserted, and then plastered over by us.

The walls are plastered so far and no, we are not and were not allowed to do anything ourselves in the shell construction, only after handover can we do it ourselves.

Yes, the developer or general contractor is not ideal, but if we hadn’t done it that way, we still wouldn’t have a house.

And no, a drywall is not possible, since the wall where the TV hangs is about 10m long and goes through the dining room and kitchen.

Thermal bridge: Only affected if you touch/ remove something from the insulation.
Statics: Only affected if you take something from the stone.
Would it be possible to present the floor plan?
I stick to the drywall option. Possibly then not the 10m area but only around the TV and, as said, additionally expand it for eyecatcher use.

A (presumably perceived as useless) interjection from the old building front: Take a white flat cable duct that lies on the wall and that's it. Nobody is bothered after 2 days...

I often wonder how perfect some things have to be.

So I wouldn’t call the thickness of the insulation the wall thickness. A 15 cm exterior wall is very thin. Structurally it might still somehow work, but it’s not reasonable.

That would already be too thin for me as an interior wall. It can’t be changed now anyway.

It will be difficult to integrate a cable duct into the wall. As I already wrote, at most I would take 1 cm from the wall. No matter how wide. Then actually imagine a drywall.