I don’t see the need to bring De Gea back to Manchester United. The decision to part ways has already been made, United have to move on. If Onana is no longer m good enough for the team they can sort other options elsewhere, I believe De Gea is not the only good keeper available to Manchester United.
If that can be done I think it would be much better because after all De Gea understands Manchester United very well and he is one of the players who is quite loyal to the club even though Manchester United eventually let him go.
The goalkeepers available at the moment are not showing their best abilities at all and Manchester United also need to buy some other important players, with the financial ability available, buying De Gea back is much better because the price is a little cheaper.
Some news I read Fiorentina are also planning to extend De Gea's contract for one year and that would be a much better decision because then Manchester United might not have the opportunity to sign him back.