Can I renew my retirement visa in Thailand before renewing my passport, and use both for travel?

You will only be extended until the end date of your passport i would renew it now

Nope that's not gonna work out if you're talking about a year extension at the immigration office inside thailand

IF you apply for a new yearly extension at the immigration office but you have a passport that isn't valid for a whole year you ONLY get the extension for the validity of the passport 😮

So in your case if you have an extension that runs out in July and a passport that runs out in Sept, you'd get an extension until Sept

SO what you'd do is renew your passport NOW, go to the immigration office and get the stamps transferred and then apply for your new extension in July so you can get a whole year extension on the new passport