Sourdough Recipe Fail: Twice

your hydration is extremely high hydration.  The author of the recipe notes this fact as well and suggests working at lower hydration and working up towards this (87%).  the only bread that I ever back that even approaches that hydration is ciabatta and as Hamelman notes in his book "(ciabatta) requires special handling (like locking all the doors so the bakers cannot run from the exits)."  I would lower the hydration as per the author's suggestion and work your way up.  It might be that your particular choice of flour brands requires less water.

I read somewhere that all purpose flour can take less water than bread flour. So that could be part of the problem.

Don't give up, you'll get there :)

which tends to matter more as you raise the hydration %. Consider that, for example, the difference between 11% and 13% gluten isn't simply 2%. If we just look at the gluten, there's 18.2% more gluten in the latter. 

In addition, if we assume a nice round 1 kg of flour, that's 110 g vs. 130 g. The 110 g may work fine at 75% hydration once we're used to it, but when we raise it to 87%, we're asking the same 110 g of gluten to hold together 1890 g of dough (2% salt) instead of 1770 g. Given that 75% hydration can require a lot of trial and error before we get used to it, it stands to reason that as we push the hydration higher, the loaves will continue to call for more and more skill.

And if we've jumped up to 87% rather than approaching it gradually, we may have made the learning curve even more difficult.

to develop gluten more thoroughly, and reduce the water to 700g (or 350g since you're doing a half recipe).  This is only a couple spoonfuls less water, so your dough will still be pretty wet -- wet enough that you can do slap and folds without the risk of overworking the dough.

If you've made lower hydration loaves before, look for that bouncy, round feeling in the dough to tell you when to stop.  It will be tacky but it should sit up for you nice and tall.  

The first time I tried slap and fold on a Tartine loaf, it took me 15, 10, and 10 min. with about half hour rests.  I'm more efficient now, and 10 + 5 min. is enough followed by a few stretch and folds.  Don't pay attention to my times though, developing dough strength depends on your flour, levain, your hands, and lots else.  It might take you longer, or you might be done sooner.  Trust your instincts.

I agree with those above that your flour is probably weaker than what's called for in the recipe, but you should be able to get good bread with a bit less water and a bit more sweat equity.  Once you get an outcome you're pleased with, you can try raising the hydration.  Try 360g water on the next attempt, and so on, until you're satisfied.

I'm currently reading Daniel Leader's "Local Breads", where he discusses many of the traditional breads of Europe and how they are made. Consistently, he says that high hydration doughs (and many of these are lower than the hydration you're working with here) need to be mixed quite vigorously. Many of them are developed in stand mixers at quite high speeds for 10 to 15 minutes! He says that, when mixed and kneaded by hand, some wet doughs need up to 45 minutes to develop into that strong, stretchy, glossy dough that will provide the best oven spring. Apparently many of these wet doughs will still spread when shaped but the oven spring is supposed to be enormous. I will be trying this long mixing myself; it sounds intriguing.

Yes, he talks extensively about the grains and flours in different regions. He often advocates using a lower-protein flour, or blending bread flour with all-purpose to get similar results. I have done this with a couple of my recipes and it ends up with a more tender crumb and crust for sure.

The doughs he's talking about are very wet (he says they look more like batter than dough when these artisan bakers are mixing them) and do result in the very webby, open crumb that you get in a ciabatta, generally.