Yorkies are harder to housebreak, but I'm a little concerned that she's literally going every 5 or 10 minutes, that's not normal. When the vet checked her, did he actually analize a sample of her urine? that's the only truely acurate way to check for a UTI and they are somewhat common after spay surgery.
If he did and she's clear, do you give her enough time outside to finish? Some puppies squat to go but then get distracted and don't finish. Does she still mess in her kennel? and if so is it just pee or poop also? I know you said you don't believe it's anxiety, but our Akita seemed very happy and was completely healthy and she would make huge messes in her kennel up to 11 months old even if we were only gone for an hour - we determined seperation anxiety, gave her mild medication from our vet and worked on modification training and with a few weeks she stopped and never did it again. It's not normal for them to go in their kennel, so she either has anxiety that you don't realize, a medical problem such as a UTI, or she was left in there too long.
What do you do when she does have an accident? You should clean up the mess with paper towel and take her and the towel outside and put it on the ground to show her "this is where it goes". Make sure she gets TONS of praise when she does go outside, but don't repremand harshly when she goes on the floor, just quickly get her outside to finish and really praise like crazy.
Sounds like you know what you're doing based on everything you mentioned
, just throwing some things at you that you didn't. I'd definatly rule out a UTI first though (with a urine sample specimen) if you haven't already because it does sound like you're doing everything right and that's really frequent.
Good luck