Thank you for the many responses,
For the niche definitely use corner strips
For the rest, I would place the first tile in the middle and compensate the few millimeters up and down. Due to the silicone in the corner, the offset is not too noticeable.
I have definitely planned aluminum rails.
To compensate: How is that supposed to work? I have the wall finished on the right, with 29.7mm tiles and 3mm joints. The black tiles are 29.9mm, so bigger! Then I would have to make the joints of the black tiles thinner but with only a 3mm joint minus the "larger" difference, it would basically be a joint smaller than existing. If I try to make the joints roughly flush, the joints on the black tiles become about 1.5mm. That is okay. The white tiles on the left are then bigger than those on the right -> The left white tiles have a different joint width than the right ones.
I don’t believe these MM differences unless it is second grade goods or something. Sorry, you are actually not doing any technical thinking here, which should have happened beforehand.
Therefore, just choose a contrasting accent tile and that’s it.
Well, the tiles are from a specialist store where the first tiler sent us to choose. I can also gladly attach a photo of the box; it even explicitly says 29.7 - 29.9mm (always a 1mm difference) with the designation.
My only stupidity, and I admit this, was to compare only the color of the tiles, but not the size! If I had chosen the larger one with 29.8 in advance, it would have fit much better with the black ones.
I would say I have done quite a bit of thinking beforehand. The niche is planned/built exactly so that the bottom edge is flush with a joint and the top edge exactly at the height of two tiles plus a joint. The slope of the screed is also considered as well as the height of the enclosures. All that would have worked if the tiles had not run out on me or if I could get another size.
The old tiler wouldn’t have put so much thought into it anyway (they don’t need to at the moment anyway, thanks to more than enough orders...)
You can of course also make the back wall from one tile (then you just bought the two square meters for nothing) and make the joint on the left wall 1mm smaller. If you grout it light, I think no one will notice except yourself.
How will the floor be??
Yes, that was already my thought. To ease the problem, I have to prevent having a continuous joint on the back wall at the same height as the side walls. Then no one will notice that the tiles on the left are minimally larger than those on the right and that the joints aren’t exactly flush.
If I take a very large tile for the back wall or even just one, then that fits. However, then the question arises, can I make the cutout for the niche properly and on the other hand, it is not quite easy to glue it (although I could possibly ask our neighbor, a tiler, two houses away for help).
By the way, the floor should get the same tiles as the back wall, so the black ones.