Tiles on a half-spiral staircase - who has experience with that?

I would have done it this way in retrospect: always 60 cm inside, as it is with the straight steps. And the joint on the front side flush with the adjoining joint at the top of the tread.

You probably can't change anything unless you spend money

Continuous is nice. At least harmonious. Alternatively 2 joints, 15-60-15 cm for a 90 cm step?

Complaining costs nerves, not money. Or was that an illegal worker?

In which world would a continuous center seam look good?

Your second approach is more like it.