I'm an ex keeper too and I dont think he gets to play for for 4 massive clubs with fundamental falls in his technique as you described because that amounts to conceding goals more so than any other position.
His style like many non-European goalkeepers is different, not as pleasing on the eye. Brazil have two of the best now, but others before them very similar to him stylistically. I think non-European keepers especially the African keepers without generalising, are more explosive in how they keep the ball out the net.
I think whats going on in front of you has a massive impact on your decision making. A steady back four, rather than second guessing what players are doing, you know how the defence reacts to different situations.
I think when you arrive at PL, like every other position but arguable even more as a keeper, he wouldnt have played in a league like this where you are either constant under attack or your watch a team low block then quickly counter attack. We have seen him take up some strange positions when teams counter attack. Certainly wouldnt have faced this many crosses.
More than anything else, I think hes trying too hard to prove he can fill the gloves of DDG. Im sure no one would have thought the skinny white lad from Spain would end up being so great. Onana needs to be given more of a chance than half of a turbulent season of uncertainty in general.
Goalkeepering in this league takes a while to settle whilst you are coming to terms with it. We have a lot of new keepers who are making lots of mistakes. What I like about him is hes mentally strong and doesnt come across like the clubs to big for him.
Patience is needed even if aesthetically hes not as pleasing on the eye as DDG. He doing something right to have the second highest save %, and I think some really top saves are going under the radar. The save against the Villa at 2-1, where his weight was on the other foot, was outstanding. There was a save from a point blank header as well but I cant remember the game?