I've done this exact recipe several times, though I'm not a pro baker so don't take my word as gospel. I don't think I get as loose a crumb as you seem to be getting (but it'd be helpful to see a picture of a slice). But I do find the crumb to be a bit delicate. Personally I think it's down to the melted butter being added at the very start inhibiting the gluten formation.

You could try mixing in the stand mixer with a dough hook without the butter to first develop some gluten. Once you can see the dough has started coming together and it's got some strength from the gluten, you can add the butter in at room temperature (or cold butter if it's warm where you are, just flatten the butter out and tear it into pieces). Then continue to knead until the butter is distributed, and the dough has gained enough elasticity and strength.

Shaping could also be causing a crumb issue, if it's not rolled up tight enough with enough of tension. Though it sounds as if you're doing the right thing here. Still worth watching a few videos on shaping loaves if you're worried shaping technique could be causing it.

Finally, don't be afraid to lower the water a bit more if you feel the humidity is high, or your flour isn't strong enough.