It seems large time signatures can only be placed relative to a group (it would be nice if they could instead be anchored to specific individual staves, is this perhaps planned?). I don’t really want the bottom instruments as a group (or at least not with a bracket). Is there a way to hide just this one bracket (except the sub-bracket for the piano) and still keep the time signature where it is? Or any other approach to achieve this?
Good news is that you no longer have to associate groups and brackets — in Dorico Pro>Engrave mode, you can change the bracketing as you wish. Could that help ?
I know that I can adjust the brackets independently (and have done so in Engrave), but the large time signatures are still related to the brackets, and if I, for instance, only make a bracket for the piano, it will show the correct time signature, but reduced to cover only the 2 piano staves.
The answer to this problem would probably be to be able to hide a bracket from showing (while still showing the latrge TS), or hide a (smallish) TS on individual staves.
Doesn’t seem possible, or …?
There is certainly not total flexibility over where the time signatures should appear at present, and I don’t rule out changes in this area in future. For the time being you’ll need to make the best of the options provided; in particular you might find the option to treat all instruments between brass and strings as a single family, so that e.g. percussion, harps, pianos etc. will all share a common time signature. That option is found on the Time Signatures page of Engraving Options.