Can I apply for a yearly extension as a spouse to a Thai national with only 5 days left on my visa?

you're fine, you need to have the 400K baht in a thai bank account in your name only for 2 months before you apply for your extension. There's no rule that says you need xxx days left before you apply. you can apply right up until the day your current stamp runs out

You need to make sure you get the documents, copies, photos, etc right the first time because you sure aren't leaving your self a lot of time IF you need to try again.

Make sure you go to the immigration office and get the most recent hand out and have your thai wife ask if there's anything NOT on the hand out that you need. last thing you do is need to mess up the first try and scramble to get required documents for another go at it