Nice result!
Is that a whole grain flour or 00, it looks kind of dark to me.
For this kind of dough I start at the lowest speed (to avoid the flour escaping the bowl), after a few seconds I up it to middle speed, and when the pumpkin has formed I up the speed to 220 and start adding the remaining water.
You can certainly overmix but at 80% it's getting more difficult..

It does need some kneading from the mixer, you'd notice that you've overdone it, if it turns to slush. You also see it once you have the dough on the bench and the skin rips when you try to fold or form it.
If I've done the dough well, I'm happy if it only takes a few folds and some pirlatura to close it. Pirlatura being the Italian word for chasing a dough around the bench with a scraper in order to tighten the skin and make a boule. IME, if the dough is well done in the mixer it just needs a couple of folds and some pirlatura to close it and make a boule that holds it shape for a long time. If it's not ready you'll feel it in the dough, and you'll have to make more folds to give it strength.