I have no idea how good Bulldog gym Balmain is, they don't have as many well known fighters as the other gyms, however that's not saying that they don't have good training, it seems like a good environment.
Bulldog Gym Castle hill has more Muay Thai fighters then Bulldog gym Parramatta. However not much cross fit options at Bulldog gym Castle Hill.
Bulldog gym Parra has good Muay Thai training, it's hard to hold as many good fighters that Castle Hill and FF Blacktown has because a lot of Bulldog Gym Parra is emphasized on other martial arts as well.
This gym has more cross fit training then you'll need, family like place. First class is free.
The Muay Thai trainer and gym owner is Tim Fisher, he's lived and trained in Thailand for many years and is a good trainer.
Ben Sandejas is a good fighter and trainer from this gym, as well as Joep who has fought Kickboxing in Holland for many years.
That's his facebook and it seems he's been to Castle Hill gym with James Tehuna as well, don't have much info on where abouts he trains now.
Full Force Blacktown.
Is the friendliest environment, but there's no two ways about it, those guys are bangers. Go there if you want to learn only Muay Thai. Free 5 day trial https://www.facebook.com/fullforcegy...35880406618096
Unless you want to truly get as good as possible in Muay Thai, or train hard, see how you go and maybe fight Muay Thai in the future, I would recommend BG Parramatta, is priced slightly higher then the other gyms each month, but that's because they have more trainers then the others, that covers other martial arts. Another thing you can take into consideration is that if you have a banged up leg or something then you can practice other martial arts, which could be the case if you kick the heavy bags enough times on the shins (recommended).
If you do trial some of these places, try and choose a place that emphasizes least on bag work.