YMMV but any issue you have with Immich that may lead you to migrate to NCM you will find again there,IMHO. Update are easier but lots of other stuff are very much work in progress, even when it works well.

My criteria is "when I want to show my vacation to X on the TV at a dinner 6 to 12 months after setup is it reliable or does it always need some tweak or whatever and ruin the moment".

I dont know the apple ecosystem, for me Google photos is king but I want to own and host my data, Photoprism is my current goto.

I would like to switch to Immich, but it is still unclear to me whether my existing directory structure will be preserved or not. In the beginning, Immich used his own structure, which is a no go for any photographer.

Immich supports customizing the folder structure[1] or using an external directory[2].

[1]: https://immich.app/docs/administration/storage-template

[2]: https://immich.app/docs/features/libraries

But that's only a half-baked one-way functionality, since the set of variables for the template are minimal. Immich can't intake my existing folder structure, or am I wrong?

This is only my opinion : it will work, but you will constantly feel like you're fighting against it / the expected way to do things.