it's not illegal to use an agent and it seems many people do, but to use one to do something against the law is illegal.
The laws are there for a reason, but for example some people use an agent to bribe an official for things like gaining a Thai driving licence or not depositing the required funds in a Thai bank It's up to the individual if they want to do this, but if the immigration officer is investigated, all of their illegal visas are cancelled (this does happen, i have seen posts from several people complaining). It's not worth the risk imo, that's why I mentioned it, in case you were unaware.
Do you mean you might need an agent to circumvent the 800,000 baht requirement?
If you are buying a house in Thailand it does not seem a good idea to break the law and risk being deported/potential banned.
I don't think you will need an agent to extend your stay otherwise, as it's a simple process.
If you decide to get married that is something I would consider using an agent for, as they can cut out a lot of the legwork and return journeys to Bangkok. Good luck with whatever you decide.
youch! TiT it's amazing the different responses you can get simply depending on the individual at the bank. It's definitely possible to have 2 accounts at the same branch, because I have 2 and I didn't need to provide yellow book or work permit.
I did this a couple of years ago and kept the 800k in the same account. I explained what I wanted to do and the reasons and they cancelled the card for my original account and gave me a new one for my new account instead.
Having said that, I never use the debit card these days anyway as I always use the cardless withdrawal option on my phone, so I kind of wish I had opted to not have a card (if that's possible).
Moving your money is fraught with potential disaster, so personally I would try to avoid doing that.
I have moved provinces twice and not transfered/closed my account as most people seem to think you have to.
It is true that they refuse to transfer your account, but the new province has in my experience been willing to provide the documentation for the extension of stay at immigration.
Of course TiT, so I am not saying it will work everywhere.
You are given 60 days visa exempt when you arrive and can extend this by 30 days if required.
The day you arrive counts as day 1, so be careful about how many days you stay, as I see a lots of people posting that they calculated the stay incorrectly by 1 day.