De Gea's age must be taken into consideration even though we can see that his performance is very good at Fiorentina but his current age is not too young anymore even though this does not have a big effect because he is a goalkeeper but still Serie A and the EPL are clearly different so it would be better if De Gea stays in his current place where Fiortentina has considered him very good and Manchester United must look for other options to replace Onana's current role.
Indeed, cooperation is still possible if both parties agree to this but of course with the awkward situation a few seasons ago I think it will be very difficult if in the end both parties (Manchester United and De Gea) can be fine later when this possibility occurs.
What Manchester United needs to do is try to make the best changes so that they can appear much more consistent and maybe the goalkeeper is one of the problems they are currently facing. For a short-term solution De Gea is still a priority compared to the two goalkeepers that Manchester United currently have, but if you want to make changes in the long term it is much better when Manchester United looks for a goalkeeper who is younger.
De Gea is still performing very well at Fiorentina and in every game he does something very extraordinary. But we also need to look at it realistically and the Premier League with Serie-A is very different so it is not certain that De Gea will perform better when he rejoins Manchester United especially if the defense is like it is now.