Why do my English muffins have such a tough crust? I'm new to the baking of English muffins. I follow the recipe for sourdough English muffins published on YouTube by Culinary Exploration. After accounting for the 100% hydration of the starter, the recipe calls for about 62% hydration. The procedure involves pre-baking in an open skillet to form a light crust followed by oven baking to finish the center. I use Sir Galahad flour from King Arthur, which contains 11.7% protein.
My EMs taste great and the crumb is exactly what I want, but I get a paper-thin outer crust which is very tough to bite through and chew. The crust seems to be the same on both sides of the muffins; i.e., there's no distinction between the side that was exposed vs the side that was against the parchment paper during proofing. Lowering the heat of the skillet and the oven has no discernible effect, other than reducing browning. The crust does soften a bit with storage, but it's still too rugged.
I fear that changing hydration will alter the thickness to diameter proportions of the muffins. Any other suggestions?
I strongly suspect that it's the trip to the oven that's making those muffins tough.
My personal recipe for sourdough muffins does not involve baking them; they are cooked entirely on the griddle.  I checked three different sourdough muffin recipes (1, 2, 3) and none of them used the oven either.
So my suggestion is that you try cooking them entirely on the stovetop and omit the oven.  This will require longer cooking on the griddle to make sure they're done all the way through; I suggest checking with an instant-read thermometer (should be 90C in the center).  I don't suggest lowering the griddle heat, which should be around 175C ... cooking them longer may actually toughen them.
The second thing I notice is that almost every sourdough recipe I've checked has some kind of fat in it, usually butter or whole milk, and the recipe you linked does not.  The recipe I've tried that doesn't is Arizmendi Bakery's, which are also very chewy and tough to bite into (I like them, but I suspect you wouldn't).  So adding some kind of fat to the recipe would probably help your crust texture; it generally does with bread.
There's other general tips to peruse, just in case you're running afoul of something else like using way too much flour during kneading.
It's also true, though, that different sourdough cultures can result in chewier (or less chewy) crusts.  So if eliminating the oven step and adding fat doesn't work, I'd suggest testing a non-sourdough recipe to see if you're having the same issue there.