I agree, but I'm not necessarily sure Martínez is available. He has a contract until 2024 with Inter and they seem to have enough money to keep him. And Barcelona, who seems to be the great contender for Martínez, is out of cash.
I stated this quite a few times but for me the plan would be:
Summer 20: Don't sign anyone, Agüero and Jesus will do.
Summer 21: Agüero leaves, we sign a lethal old striker on loan (Vardy?).
Summer 22: We sign Mbappé for free (obviously start the charm operation to bring him as soon as possible).
If Mbappé is out of the plans then:
Summer 20: Don't sign anyone, Agüero and Jesus will do. Sign a CB, a LB, a DM and a CM.
Summer 21: Go all out on a striker (Haland seems like the only amazing option).