Guest bathroom 75x90 cm shower fixture placement

So I would also put them on the short side, because otherwise, as someone already wrote, you only make the shower even narrower...

Yes, there will be tiles on top, but they only add about 1cm, so from 76 to 75cm down.

Are you only putting tiles on one side? Are they screwed to the wall?
Tile 1cm + adhesive at least 0.5cm times 2 sides = 73cm.
And then a fitting with a rod that easily takes up 8cm width?
I would definitely have the shower mounted on the narrow side as well.

+1

Fitting and showerhead on the narrow side. Otherwise, it will be too tight.

Wall tiles are usually about 7cm thick (otherwise they would be much too heavy and would not hold). When applied with a 5- to 8-tooth trowel, it results in an adhesive thickness of 2-4mm (tile pressed on, trowel at a 45-degree angle, which uses only about 70% of the nominal tooth depth).
It doesn’t really matter if it’s 1cm more or less, but as the unanimous opinion probably is, definitely place the fixture on the short side. You can’t enlarge the shower? 75 x 90 is very, very small anyway and should really only be seen as an emergency solution.

Unfortunately, it can't be bigger because the bathroom itself is not large and there also needs to be a sink and a toilet installed. If I choose the option as marked, the faucet would at least be out of the way. I would like to have the shower head on the longer side so that the water does not spray completely outside the shower. And hopefully the bar will take up less space than the faucet and is supposed to go towards the corner.

Wandfliesen sind in der Regel ca. 7cm dick

I do hope 7mm

Yeah, rather 7mm
You can also place the fitting in the middle of the short side and the actual showerhead in the corner.