HFD sounds like king county medic one. Least while I was there.Yeee-Hawww!
In all honesty, it’s where you feel comfy. Texas has a lot of high-functioning services doing cool things...and they’re far from perfect. And a lot of services in CowboyHatLand are terrible. Like, super-terrible. CA is actually probably better overall when you consider HFD has BLS crews running unstable cardiac patients for lack of medics.
As far as what to expect- that will vary from county to county. Also, I don't know what an Australian paramedic equates to scope-of-practice-wise in The States, let alone California. Perhaps give us some more background info like why you want to move to California, or how long you've been a paramedic in Australia, etc.
The link above is truly your best bet because, honestly, I have no clue as to how difficult the transition would be coming from another country. I would also caution you that in comparison to even Australia, California can be very restrictive with individual county scope of practices; even still, it's universally one of the more archaic states.
Many of California's fire services provide paramedic care as well. So, in my opinion, the best place for you to function in an environment in California that might be similar to what you are used to would probably be the HEMS industry. But, even this area of EMS can be quite competitive here.
And before @RocketMedic comes "yee-hawing" on in, there are other states to consider (like Texas) who offer ground providers options similar to what most HEMS paramedics in California are doing.
The only other things I have tried to look into but have had a hard time is applying for a Visa that would allow me to live and work in the US but I think that's a seperate challenge in itself.!