Hi @florian-bergmann, revisiting your project. I tried several things, and here the result.
I duplicated the flow and assigned flow 2 to a new Full Score (corrected) layout for clarity and to compare.
(I also grouped the players in 2 groups in Setup, so you don’t need a Bracket change)
Working in Full Score (corrected) layout,
I changed all the Time Signatures of the 1st group to global (some of them are also globally used by the 2nd group), setting some of the TS of the 2nd group as hidden. This takes care of the score and the first 4 parts automatic casting off (always on the barlines for the 1st group). Here my changes (as comments):
For the other remaining 4 parts (x.1), since many local TS remain, inherited from the score, manual system breaks are necessary (with some further moving of the resulting signposts). I also used Propagate Part formatting (Include system formatting checked) for the 1.1, 2.1, 3.1 layouts, so it goes much faster.
(I optionally polished the score layout to be on 4 pages)
Here the resulting Dorico file (see at Full Score (corrected) layout, and all the parts layouts. I hope it can be of some help: