Need help on how to adjust to avoid dense/heavy loaf

Hi everyone! I was wondering if I can get help with a problem. I think my bread is underproofed, but I'm not sure whether this is a problem during the first fermentation period or the second. I'm not a novice baker, but it's been about 7 years since I baked bread regularly, so any suggestions for resources on a baker who needs a refresher course in bread-making skills would be welcome.

Pictures to illustrate the problem can be found here (sorry, I'm having trouble with the photo upload functions)

TLDR:

I'm pretty sure my bread is underproofed, but not severely underproofed because the crust color is quite stunning and there was some oven spring. However, I don't know whether I should have increased the length of the first fermentation or the second. I don't know how to tell when either of those is done in a predictable way that doesn't vary significantly from bread to bread (e.g. some breads seem to need to double in size during bulk fermentation while others only need to rise 1.5 times in size). I also always feel uncertain about results of the "poke" test for the second fermentation (if you poke the bread and it doesn't spring back, it's ready), because I think I've accidentally overproofed some breads when I've relied to heavily on it.

Full story:

Back when I used to bake a lot, I made a lot of the standard Country Bread and Whole Wheat bread from Chad Robertson's Tartine Bread Book. However, because Tartine often proved time-consuming, I sometimes made a variation on this bread (Pain de Campagne), but replaced the poolish in the recipe with starter. I still used the 1 tsp of yeast to quicken the process and the starter gave a richer flavor than the poolish. However, I don't remember whether 

I made my best guess from what I could remember

• 235 g All-Purpose Flour
• 25 g whole wheat
• 190 g Water
• 10 g Salt
• 1 Tsp. Instant Yeast
• 435 g sourdough starter (at 100% hydration)

I had massive amounts of trouble developing gluten when I mixed it. I realized, fairly quickly, that I had made 2 mistakes:

(1) I didn't change the hydration percentage of the starter to match the hydration percentage of the poolish (which is 64% in the recipe. Big oops. 

(2) I was using regular all-purpose flour (NOT King Arthur)-- and the King Arthur flour has higher protien content (by 1-2%) than my current all-purpose flour.

I added a tiny pinch of salt and a lot more flour. It helped, but I was still having a lot of difficulty. After 40 minutes of folding (folding the bread several times, waiting 10 minutes, and then folding again-- not continuously folding for 40 minutes), I was fed up and put the dough in the refrigerator. After a few hours, I folded a few more times (better but still not there). Finally, before I went to bed, I sprinkled a little bit of vitamin C over the bread, folded it several more times, and then put it back in the fridge.

This morning when I woke up, the dough passed the windowpane test (it had been sitting in the fridge for about 16 hours by that point). Rather, it guess it came close to passing the window pain test, because I could stretch it and see light through it, but it ripped fairly quickly. However, it was close. I folded it twice more, and let it sit out for an hour to adjust to the temperature of the room.

After that hour, I did a pre-shape, let it sit for 30 minutes, and then shaped it and put it in a banneton. It spread, with the preshape, but didn't "drip" or become very thin on the edges, so I felt that the gluten was moderately well-developed. I let it rise for another hour and then baked it.

Thoughts and strategies for how to tell when bread is done proofing would be greatly appreciated. Thanks!