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.
". . . renewal is not guaranteed . . ." Look, nothing is guaranteed, neither is your O visa guaranteed. If you plan to be an expat then you have the accept the impermanence and uncertainty inherent in that status. Yes, I am going to plan on my 10 year LTR status. Long stay visas change a lot less than tourist visas.
If Thailand actually does tax worldwide income, like the US already does, then that is what I will deal with. My Social Security is protected by the Dual Tax Treaty. I will remit my work pension to Thailand and it is exempt with the LTR visa. Any work pension left in the US would be a small amount and I will pay the tax, if any, on that small amount.
For health care I have a Thai policy and savings in the US set aside for that purpose.
As for meeting the requirements, the BOI decides, not you. If you want a discussion on that then you are going to have to reveal more information on that.