I am looking for some help troubleshooting a sourdough loaf/upping my game!

Hello fresh loaf! I am pretty new to baking proper bread - I started making the classic Jim Lahey no-knead bread halfway through last year, and after experimenting with various commercial yeast preferements I have started the new year with a resolution to bake good sourdough bread, and to never buy a loaf from the supermarket again! After a couple of weeks my starter was nice and vigorous, and over the weekend I made my first loaf with it. I have used the overnight country blonde recipe from Ken Forkish's Flour Water Salt Yeast book with a couple of minor variations. The variations are as follows (justification in brackets).

• My starter is 100% wholewheat and 100% hydration (100% hydration makes calculations easier, and I prefer the simplicity of not mixing flours when feeding daily. I might switch once I fridge the colony and feed weekly).
• I went for 75% hydration instead of 78% (I have a hunch that UK flours are not as thirsty as US ones, since I have much better results when I slightly drop the hydration of FWSY recipes when comparing to images in the book and the behaviour of dough in Forkish's youtube vids. I haven't yet worked out exactly how much to reduce by, so still experimenting).
• I made one loaf rather than two - i.e. halved the quantities (I don't have a large enough container for a 1kg flour dough!)

All other aspects remained the same, and I adjusted the water and wholewheat content of the recipe to match my different starter composition. I built my levain with 10% mature starter, 45% white flour (strong bread flour - 13% protein) and 45% fresh water. Otherwise, recipes was the same - mixed for 10 minutes, and put in 6 stretch and folds over the first 2 hours of an overnight bulk fermentation (in my kitchen that maintains around an 18C temp all night, rising up to 20C in the morning depending on breakfast activity...).

In the morning the dough had doubled in size (seemed good!) but I noticed some large bubbles on the surface. This suggested to me that I hadn't built up enough strength in the dough (or that it was overfermented? I don't know much about over-fermentation...) - it felt like (as in, I just had a hunch or a feeling...) that bubbles had been rising through the dough. I pressed on to shaping - the dough felt a little slack but I feel like it tightened up into a ball fairly well - and then into a banneton to proof for 3 hours. I noticed, again, over that proofing time, that there were large surface bubbles appearing and that the top was not forming a dome, rather the dough was rising and spreading into the banneton. After 2.5 hours the dough felt ready to bake, so I just went ahead with turning it out onto the surface to get into the dutch oven.

I will be referring to some photos - they are in this imgur album here https://imgur.com/a/098zuC8

When I turned the dough out it spread a lot (and stuck a little to the banneton) - rather than retaining some of the banneton shape it spread into a disc fairly uniform in height. Still, have to try these things, so I dumped it into the dutch oven and baked. The loaf looked a bit odd straight away - there are noticeable hollow bubbles on the crust (first and fifth image) You can see in the second photo that the loaf picked up a lot of the shape of the dutch oven - probably because the dough was loose.

Once the loaf was out I waited until it had totally cooled, and cut the swizzle to taste. The crumb looked nice at the edge of the dough (see where the swizzle was taken - third image), but there was clearly a problem with the dough buckling into the dutch oven. The dough had spread to be wider than the dutch oven, hence the fold of floured dough-bottom inside the loaf. Then I halved the boule to check the crumb out properly. I am happy with it as a first attempt, but it is a little dense - although I got more of a rise than I suspected I might after turning out the loaf! I suspect that is due to the constraints of the dutch oven. The flavour is really nice - mildly sour, a little buttery, and with a nice rich crust (a bit of bitterness, loads of dark caramel-like flavours, a mild tang of acid) so I am really happy with that. The crumb looks fine as a first sourdough loaf, but really I am unhappy with the form of the loaf and the slackness of the dough. So here are my questions...

Is it possible from these photos to determine if the problem was a) not building up enough strength in the dough, b) pushing the bulk fermentation too far, or c) my shaping technique (I am, for the time being, following the technique in FWSY) or d) too high hydration? And then as a follow-on question, what kind of techniques can I try to improve this? My hunch is that is either the dough strength or over-fermentation (since 75% hydration seems reasonable even given transatlantic flour differences). I can't find much information about over-fermentation online - what are the signs of this? And from looking at other recipes it seems like six stretch and folds is around the amount recommended, or even slightly higher, for a lot of recipes. I am a bit stumped! Still, the loaf tastes good so I am sorted for breakfast for a few days until the next bake!

Also, FWSY mentions the seam of the dough opening up in the oven, but I find that this doesn't really happen with longer prooves - it seems to seal shut over time. I tried to slash the loaf with a hobby knife with a fresh blade in (all I have for now) and the dough just stuck to the blade and rumpled up, leaving a broken slash (like a dotted line) behind - that's why the top looks so messy and weird! It's a mixture of cack-handed slashing and the seam opening!

Anyway, I am rambling on! Any tips and techniques are welcome, as is any more specific diagnosis of the problem based on my images! Thanks freshloaf!