Finale Maestro Text doesn’t include any of the accidental characters that Dorico expects to find in a SMuFL-compliant font that is suitable for use by the Chord Symbols Music Text Font font style. I would suggest you leave that set to Bravura Text, and instead substitute individual symbols from Finale Maestro Text where necessary.
If you set the font style to use Finale Maestro Text, you will typically see fallback symbols, because the chosen font doesn’t contain the necessary symbols. It does contain the first six characters from the Chord symbols range, but none of the parentheses or the altered bass slashes. It doesn’t contain any of the symbols from the Standard accidentals for chord symbols range. So when Dorico constructs a chord symbol and it wants to use any of these symbols, you will instead see a substitute from another font.
As a result, I recommend you specify Bravura Text as the font for Chord Symbols Music Text Font, so that at least by default you will see a symbol from a known font.
To replace the standard flat and sharp accidentals, you can do this via Library > Chord Symbols using the method I described earlier in this thread. Once you’ve replaced the standard Bravura Text sharp and flat with the ones from Finale Maestro Text (though the flat in particular in Finale Maestro Text is one of the ugliest things I’ve ever seen, and I’m really not sure why you would want to use it for anything!), you can select the chord symbols you had to create in the dialog in order to edit the accidentals and click the “revert to factory” button to remove them from the dialog.
You shouldn’t need to edit the actual roots at all: simply editing the Chord Symbols Font font style to use Arial (or Helvetica, whichever you’re using) should be sufficient. After editing the sharp and flat accidentals in Library > Chord Symbols, all roots will use the Finale Maestro symbols from that point.
Editing the accidental symbols in an alteration like b9 or #11 is still awkward via the existing editor. For the time being, you will need to edit b5, b9, #11, etc. one at a time. But the data is structured in such a way that it is possible to replace the accidentals in those components, as this is what enables you to edit the kerning between the accidental and each number; what remains to be done is to make it possible to select the accidental and the interval individually in Library > Chord Symbols.