Can my wife get a trailing spouse visa if I obtain a retirement visa in Thailand at 50 years old?

If you can I would suggest you get a Non-O based on being over 50 and your wife gets a Non-O based on being a trailing spouse in the UK before you come to Thailand. If you wait until you are inside Thailand to apply for the Non-O based on being over 50 then your wife will have to visit an embassy outside of Thailand to get the trailing spouse Non-O plus she will have to wait until you are on an annual extension to do that.

Trailing spouse is possible, you would be advised to get your marriage cert legalized for use in Thailand by having it certified as an original which is done somewhere in Milton Keynes assuming you married in UK, but please google it as things change. You may have to get it translated into Thai & certified again here. This can be done in Bangkok or maybe by an agent if neccessary. Different immigration offices interpret rules differently so always best to check exact requirements with your local office.

Come on tourist & convert to O or O visa but work fast as you will need 800k in Thai account 2 months before your 1st 2year extension

Better wait til she's 50... Enjoy being single again for 2years

She can get a trailling spouse visa on the back of your Non-O over 50 (i.e. retirement) visa. Only your visa needs to meet the financial requirements.