......more than anything, the dept of immigration want to see proof of a ongoing relationship. You obviously have that already. If you have achieved that without an agent you have done well. I cannot recommend any agents in Thailand. But I do know they need to be registered with the dept to charge for their services. Good luck mate.
Has she been to Australia? If not you need to consider getting her a 3 month tourist visa, 2 or 3 of those and then apply for a partner visa. You don't need an agent, just money and patience.
.....you misunderstood. Bangkok and Chiang Mai both have the worst air quality in Thailand. But living in or near Rayong if one needs to go to Bangkok or Pattaya, it is not long on the expressway. I have been all over the country and finally settled on Mae Rampheng beach. Quiet and peaceful with just enough nightlife. Not far and not long to the madness.
The air quality in Chiang Rai is extremely poor. You are better off to look at the coast east of Bangkok. Around Rayong is good. Not far to Pattaya or Bangkok. Good shopping. Clean air. Others can answer the visa questions for you.
MOTS services I think are 25,000 baht for visa, 4000 baht for bank account. They will cover the 800,000 thb and sort you with a Pattaya address if you don't have one.