No Oven Spring and Pronounced Sour Taste

First of all I don't really mind the sour taste, in fact I like it. However, on my last attempt the flavor was not well rounded. Also as my title states there was zero oven spring on my last 3 attempts. I will say that I am using what I believe to be a lively starter. I refridgerate mine and remove a teaspoon on thurs morning and feed with a 1/4 cup of warm water and 1/2 of KA AP. By fri morning I am doubling the size of the feedings (which occur every 12 hours) and the starter is at least doubling between feedings. my recipe is from Sourdough Home, it is as follows:

1/4 cup starter
5 1/2 cups (687.5g) KA Bread Flour
1 cup (125g) KA Whole Wheat Flour
2 1/2 cups water
2 tsp salt

I begin by stirring the starter into the water. I add the WW flour and mix. Then I add the Bread Flour and mix with a wood spoon until combined. I set aside the mixture, before adding the salt, covered, for 2 hours to autolyse.

I then add salt and mix with a Kitchen Aid for 5 minutes until dough is mostly pulling away from the sides of the bowl but still sticking to the bottom.

I then remove the very sticky dough and place in food grade vertical container with lid slightly ajar and leave on counter to bulk ferment at 68-71 degrees F for 12 hours. I have been making the dough at 8:00 on Saturday night and coming back to it on sunday morning at 8:00.

When I come back to the dough it has more than doubled and possibly started to recede a bit.

I pour the slack dough onto a floured counter and separate and weigh. Dividing into two boules. I shape and place in (bent whicker?) benatons. First I coat the benatons with a lot of Semolina flour to try and prevent sticking.

I let the dough rise, covered, in the benatons for 4-5 hours until they have roughly doubled.

In the meantime I preheat the oven to 375, with quarry tiles in place, for 1 hour. I bring a pot of water to boil on the stove and pour into a pan that I place on a lower rack.

I invert the benatons onto a wood peel, also well dusted with Semolina and quickly slash with a razor blade then spritz with water (Already I am noticing that the boules look like UFOs). Moving deliberately through each of these last steps I then slide the dough onto the hot quarry stones and quickly close the door.

I turn on the oven light and watch as nothing happens for 45 minutes when I finally remove the boules, which have not "bloomed" at all. The crust is light brown and crisp for a few hours and the crumb is decent, not completely open but not super dense either.

I just purchased a Sassafras Cloche and intend to use that going forward.

Thanks for any input you folks might have for me,

Scott