I would apply for the visa yourself either in your home country or in Thailand. From what I hear, opening a bank account has been increasingly difficult. You might want to look for an agent just to open the bank account which is a one time thing. After that you would be free of the agent.
I opened a Bangkok Bank account in Kantharalak last month. It was easy but I had a yellow house book which might have made a difference. It took less than an hour and they were very friendly. I’ve got an American passport which usually deters banks but it wasn’t a problem for me.
The Sisaket immigration office has a reputation for being very helpful.
Currently you can enter Thailand 60 days visa exempt and you can extend that for 30 days. You should confidently be able to exit and repeat. When you exit stay a few days in Cambodia since you plan to go there long term anyway. Don't try to exit Thailand and return the same day. I don't know exactly what would work but that is the least likely.
Your profile says you are retired and in the Philippines. Thailand does NOT have the same policy as the PH regarding tourist entries. It is easy to stay in Thailand as a tourist for 6 months but after that your stay becomes more uncertain (at the discretion of the immigration officers). If you are an actual tourist then that isn’t so much of a problem since you are in a position to leave at any time. But if you invest in a yearlong condo lease, buy a motorbike, or fall in love then you situation is more precarious as a tourist. In the PH you can be a tourist for 3 years and do a border run and then repeat. Thailand doesn’t have anything like that for tourists.
A retirement visa in Thailand will operate similar to the SRRV in the Philippines but it will be more expensive. I would think that even using a visa agent in Thailand would be at least as, probably more, expensive as living as a tourist in the Philippines.