If you insist on using an agent, and I do so because I value my time, just go to immigration and ask the supervisor to give you contact information for three good agents.
The agent will help you open a bank account as well as get a Non O visa.
Don’t pay more than ฿13,000 for the agent, visa AND multi reentry permit per visa.
You will want to have the agent AND the bank account in the same area you live.
This is why I recommend you put the money in a dedicated fixed deposit account and forget it exists.
The surety account should be separate from all your other accounts. It probably would also be advisable to use a different bank than your day to day accounts to prevent the bank from doing anything as well.
There will be naysayers who tell you that a Yellow Book and Pink ID card are worthless. However, if you have been here a while and are informed you may have a different opinion.
For example, when you apply to add your name to the Yellow Book, you are issued a 13 digit Thai ID number as part of the process.
Those that have been here for some time will remember when Covid vaccines first became available, the only way to schedule an appointment was online - and that process required a Thai ID number in order to complete.
There was also a time when your name in the Yellow Book was your pass to gain access to the country when entry to the country was restricted under the quarantine.
Then, when the country began to reopen, the government began a program of providing half off travel vouchers. That program was available to foreigners, but required a Thai ID number to complete the online application.
Finally, the Yellow Book often (but not always) eliminates the time and expense of securing a Residency Certificate from the local immigration office when doing things like buying a car or opening a bank account.
The Pink ID card will rarely provide you with Thai National entrance fees to places like National Parks, but it will make things like flying, checking into hotels and sending packages easier and more convenient.
I am not aware of any drawbacks to securing these useful credentials.
While the permit is valid for 12 months every country I am aware of requires you to apply for a license the lesser of 90 days or “establishing residence”
Established residency is typically signing a lease or accepting a employment.