Portrait or landscape?
That's a good question.
At some point, I realized the answer:
The lines decide.

Many photographs have dominant lines.
Ever since I started paying attention to them, the format often chooses itself.

Lençóis 1:
The horizon in landscape photography is usually clearly horizontal.
A clear case for the landscape format.

Mauskathedrale:
The gate is clearly dominated by vertical lines.
The subject would not work in a landscape format.

Goldlampe:
Sometimes, however, it does not matter.
Here, the only significant line leads vertically.
But it is not important enough.
The sense of space (a topic for another time) matters more.

Since I started seeing lines, I see the subject more clearly.
And I no longer struggle with choosing the format.