I wanted to respond to your first paragraph because it's absolutely correct.
The reality is he's capable of really top class saves, it's literally his only redeeming quality to be fair. That doesn't mean every stop he makes is top class. The free kick at Leicester and the Martinelli header which the commentary teams raved about are stops you would expect any of our 3 senior contracted goalkeepers to make, they are pretty basic stops for a PL level goalkeeper. He has a habit of making certain saves more difficult than they need to be - hence the free kick on Thursday, if you look at his initial position he's too far over hence taking two steps and a hop before Maddison even hits the ball, but that's ok as long as you do stop them.
Thought Arsenal's goal was indicative of a couple of the issues we face with him. Odegaard played the through ball from roughly 35 yards from goal, it is not until the ball is travelling through the D De Gea makes his move forward. He is clearly being coached to be more proactive at times which is no surprise but brings issues when a goalkeeper isn't used to it, such as the delay in moving forward, it's not his natural game so it's not his instinct to come for balls, which is why we get that delay of a split second in him moving out, we saw it multiple times last season too and often by the time he decides to come it's too late. The second is his communication which is not something that can be proven by stats, it's more than issue I have with him having stood behind him week in week out for the best part of a decade; he clearly doesn't shout that he's coming, so Dalot tackles Jesus - why wouldn't he? That's not a ball De Gea ever comes for so he had no reason to expect to see his goalkeeper coming for it, he's got his foot in to stop the initial shot and obviously doesn't expect he's leaving Saka an open goal in making the tackle.
By no means is it a goal I'm blaming the goalkeeper for but it just highlighted a couple of issues that aren't going to go away with De Gea, no matter how well he is coached; If you're going to come in those instances it has to be immediate and it's only immediate if it's instinctive which it never will be and your defenders need to know you are coming.