Though I agreed that with your opinion on Onana being prone to mistake, however I still rate him as a good goalkeeper and I believe Erik ten Hag had sticked to playing him as their main goalkeeper because of his ability to use both feet which he can use to launch a counterattack or to start build up play from the back, those are two qualities abilities he had in his possession which I doubted any of Man Utd reserve goalkeepers has those qualities, these are tactic adopted by Man City goalkeeper Ederson and Arsenal Raya which had helped the team to become very competitive thus Onana needed to up his game in term of correcting his mistakes.
Onana has made many mistakes, it cannot be denied even though overall Onana cannot be blamed for the poor performance of the Manchester United team, indeed overall the performance of the Manchester United team is not going well and the one who should be responsible is Ten Hag, he who has failed to develop the performance of the Manchester United team even though he has made many changes.
I think it is no longer suitable for Ten Hag's playing style to apply or indeed Ten Hag does not have good qualities as a coach to be able to improve the performance of the Manchester United team, so no matter how forced the results will still be the same and I think it won't be long before Ten Hag is fired.