What type of visa should I apply for to move permanently to Chiang Mai, Thailand?

Hello, I am really confused on this VISA law. I am a US citizen still living in the USA. I am 71 next month. I want to move permanently to Chiang Mai In June 2019. I understand the income requirements and qualify. Which visa do I apply for thru the Thai Consulate in Los Angeles, CA. According to what I have read is I can ship my USED personal items into Thailand Duty-Free ONCE in conjunction with my move. It cannot be accomplished earlier than 30 days before my arrival and up to 6 months after my arrival. (Can be extended). I am not married and never lived in Thailand before. The confusion is which VISA and with my Customs Import Form I must attach a letter from Immigration giving me that DUTY FREE (one-time) status. Yes, I know how much that VISA costs. I hope my needs are clear. Please help. I do not know how to accomplish these requirements. I have all the forms, medical, police clearance, customs declaration, etc. I am taking a ‘tourist visit’ in April to open a Thai Bank account and find a condo to live.