What documents are needed to renew a 12-month retirement visa in Thailand, and is health insurance required?

Fyi, you're not renewing your visa. You're extending your permission to stay. You're not on a visa while in country,but your extension is based on an initial visa,which I guess is a Non-O.

Which visa are you on now?

Non-OA requires health insurance and can be obtained only from home country. I am also collecting for 2nd renewal: Samut Prakan requires verified (stamped) TM30, as i am moving onto 65,000 monthly transfer method i am awaiting the 12th on 4th of August. Proof that I maintained 800,000 for 3 months from the last extension and minimum 400,000 thereafter until the 2nd extension application. I will also prepare schedule of transfers from Wise to my BKK a/c which seems to be the recent requirement in some IOs in addition to transfers being shown as “Foreign Transfer”. The form, 2 copies of passport including visa pages, photos and 1900 baht. A checklist for both of us. Anything else to add?

I’d just go through an agent it saves a lot of headaches 😉

The OA requires health insurance. The O does not require health insurance. You probably have an O visa.

Go to your local immigration office and get the current list of requirements for an extension

You must have got a non O visa originally, so you don't need health insurance.

If you didn’t need any mandatory health insurance for the initial visa, you’ll don’t need it for the extension either. The requirements varies slightly between immigrations and it’s best to visit your immigration and get handed over their requirements before you apply. If you don’t have a affidavit letter regarding income the 800K baht has to be seasoned for two months prior to your first application. If you’re on your second year, you’ll need to prove twelve months consecutive transfers of minimum 65K baht if you want to change to the income method.

Retirement visas are for 90 days. Can only suggest you do a lot more research and reading.