Is there a way to align all time signatures vertically, i.e. to avoid the superscription over ties/slurs?
No, Dorico will always automatically position the time signatures at the closest possible position to the staff.
I wonder why you’re choosing to use time signatures above the staff for a single-player part? That’s a rather unorthodox choice.
Because I prefer to preserve the flow of notes in a solo piece where the time sigs are somehow “redundant”. I am astonished because the superimposed time sigs look weird. It would be nice if they could appear *masked"/with a solid background on a line.