Can my wife be a dependent on my Retirement Visa in Thailand, or do we both need separate visas?

You need to show 800k in separate accounts.

We do the Non-O based on being over 50. My wife comes is a dependent on that and remains so for each yearly extension. The primary (you) has the bank account under your name only to satisfy the financial requirement for both of you. Just maintain the balance in your account per immigration's rules. Super-easy.

BWI !

I do not recommend a education visa at this time unless she is going to go to a university here.

You would both have to have separate bank accounts with your own funds for you both to get retirement visas, alternatively you can apply for the retirement visa and your wife does an ED Visa (Education) and once you have your retirement visa she can follow you, but first you have to have 3 months non O and then apply for the retirement Visa, your funds need to be in a Thai bank for at least 2 months before you apply.

First question is what are you calling a retirement visa, there are 6 or 7 visas that fall under that category.

The most common is the Non-O based on being over 50, no insurance requirement, you can get it in your home country or in Thailand but your wife won't get a dependent visa based on that so you will each need to show money in the bank etc and if it's a joint account you will need to show double the money (half each).

If you apply for a Non-OA visa in your home country before travelling to Thailand your wife might be able to get a dependent visa based on your visa without showing additional funds, but you can only get it in your home country and will require insurance and police check etc.

There are other options such as the LTR Wealthy Pensioner which is probably the best if you qualify for it but it's quite a high bar