So many problems with oven spring

Hi everyone, I'm new here. I've been reading a lot of your posts and I've found everyone so knowledgable and helpful!

As a lot of people, I started baking bread since this whole quarantine situation started, and now I've been working on my bread baking skills for over a month now. For the past 3 weeks (something around 10 loaves) I've baked the Pain de Campagne recipe from the FWSY book, with mixed results on the oven spring area (none of which I'm proud of).

It seems though, that today I've hit rock bottom. 

This is my bread from today's bake.

10:25 am - Mix levain 

Room temperature: 23.4 C

1. 50 gr 100% hydration starter

2. 192 gr water @ 32C

3. 50 gr whole wheat flour 

4. 198 gr 00 flour @ 13% protein

Levain temp: 28C

5:30 pm  - AUTOLYSE

1. 620 gr water @ 34C

2. 740 gr 00 flour @ 13% protein

3. 60 gr whole wheat flour

Room Temp: 23.7C

6:30 pm - Mix

1. Autolyse dough

2. 21 gr salt

3. 2 gr instant dried yeast

4. 360 gr levain

Room Temperature : 23.8C

6:45 PM - Slap and Fold

7:48 PM - Stretch and fold

8:20 PM - Slap and Fold

8:50 PM - Slap and Fold

9:06 PM - Slap and Fold

10:45 PM - Divide and reshape

11:15 PM - Shape into boules and proof 1 hour in counters in bannetons

Here, I'm wondering if my 10 inch inner diameter might be too big for this dough

12:15 AM - Proof in fridge

10:20 AM - Bake straight from fridge in cast iron combo cooker, 30 mins lid on, 30 mins lid off.

So, as you may see, I had to do a few modifications to the recipe:

1. As my starter is 100% hydration, and the recipe calls for 80% hydration starter, I'm adjusting the levain to create a 80% hydration levain.

2. Instead of stretch and folds, I did slap and folds, as my previous attempts had not as much oven spring as I'd like, I was wondering if this was due to poor gluten development if the bulk fermenting phase.

3. Added a pre-shape step and then an 1 hour proof in counter (inside the bannetons), so as to get a little bit of rise before going into the fridge. Previously I was doing as the recipe says to shape and then go into the fridge immediately, but I wasn't getting any rise inside the fridge (is this just me, or no one gets increased volume when cold proofing overnight?).

Any tips or guidance would be really appreciated! Thanks everyone in advance!!