So I had a idea what if Broly became Mystic through the same means as Gohan how strong would broly be?
The Mystic ability is just the old Kai releasing hidden potential (latent powers) of whoever he dances around for in a circle for a few hours (I doubt Broly could restrain himself that long), Broly is probably at is maximum or at least near it, so I don't think he would get much stronger.
Broly's power is Maximum and keeps growing.
Also he would have to stay still for 2+ years for his potential to be released lol.
I doubt the old Kai would want to dance and try to unlock Broly's power for years and Broly wouldn't stay still for even 3 minutes let alone years.