Even if the MU boss welcomes him, I think Ronaldo's decision to return to the Premier League at the age of 39 is quite risky. Premier League defenders are quick and strong, so it won't be easy for Ronaldo to score. Ronaldo should stay in Saudi Arabia, which is more suitable for older players like him.
Returning to MU is too risky and too big of a challenge. Ronaldo should not trade his honor. Keep a good impression on the MU fans. His ego won't like sitting on the MU bench. If Ronaldo comes back. I believe it will happen.
Even if that doesn't happen to Ronaldo when he is back in the Manchester United team, he will also not be played full time by the Manchester United coach even though the coach is from the same country as Ronaldo. Because Ronaldo is approaching 40 years old, of course there are many different things in Ronaldo himself so he will definitely be quite overwhelmed when facing defenders from the current Premier League team. But I want to see how Ronaldo's decision is even though your suggestion also seems wise for Ronaldo.
Cristiano Ronaldo left Europe for Saudi Arabia because when he decided to leave Europe and come to Saudi Arabia, no European club showed interest in signing him. The last season he played for Manchester United is not to say that his performance was too bad but he may have had some personal conflict with the manager due to which the manager did not give him a place in the regular eleven. Now there are new talks that Cristiano Ronaldo can return to Manchester United. Although there is no concrete information yet, from what we know about Cristiano Ronaldo, I don't think he will return to Manchester United. He is well respected and trusted by the Saudi league and he may want to finish his football career from this Saudi league.