Most custom firmware is dedicated to those gears that are widely used, so for the S9 hydro, I don't think many people have that and that explains why most custom fm devs didn't bother to make a custom firmware for it.

I suppose you can use Vnish firmware for the Dual S9 which is basically just the normal S9 firmware but would accept 6 hash boards IIRC, so if thierry4wd can't help you with the S9 hydro you could try Vnisha (Asic.to / AwesomeMiner) you can also reach out to taserz aka asic.to and ask him about it.