Make an in-wall cable duct yourself?

Where exactly is the surface-mounted duct supposed to go - up on the right side of the media devices to the TV or what?

probably from the middle of the TV down behind the lowboard

With 15 cm outer bricks? If you're a fan of Einstürzende Neubauten (no, not the 80s industrial band), go ahead! ;)

is that really that problematic?

HDMI via wireless? well, HDMI 2.1 probably can’t do that, right?

maybe an HDMI switch would be interesting where only one cable goes to the TV

does such a switch recognize which signal is currently active and automatically switch the port?

would a smaller empty conduit be unproblematic? e.g. an M25?

Turn the lowboard into a highboard, then you don't need a channel at all.

"is that really that concerning?"

Check with your/a structural engineer, they will advise against it because it deviates from the DIN. What you do afterward is your business and YOUR risk. It's always an individual matter of "could/should something happen". Therefore, I don’t think anyone here in the forum will say: "hey come on, it's all easy, knock down your wall!"

If you are willing to take a risk yourself, then do it; if not, then leave it or take an alternative route. THIS DECISION, however, lies solely with you!

(...)
You can ignore static questions. Correctly, you cut vertically, so it only affects a very small strip.

Oh, that's why there's a standard that specifies how deep you are allowed to cut into which stone.
Because it doesn't matter structurally. Thanks, Mr. Structural Engineer!

Then I can mill a 20cm deep storage niche for the TV magazine into my 36.5cm exterior wall.

If you yourself are willing to take a risk, then do it; if not, then leave it or take an alternative route.
THIS DECISION is solely up to you!

Since you have absolutely no idea yet how everything will behave in the long term in the new building... how the load distribution looks once the building and the ground have "settled," I would just leave it alone. Otherwise, it can very well happen that in 2 years your TV falls off the wall because the stone cracked or burst due to over-stressing (and weakening).

And to listen to the forum members who say "No problem, go ahead"... you have to live with the damage, not me and not them ;)

There are also TVs with an All-In cable, e.g. Samsung. Only a thin cable runs to the TV, all other connections are in a small BOX which can be placed in the lowboard.