This is the first slam where as a FAA fan I think there are only positives to take away. When he got destroyed by Thiem in the USO 4R last year and lost to Karatsev in this year's AO 4R after being up 2 sets, there were obvious mental red flags there. He folded under the pressure of the occasion. This time he came with a great strategy (attack Matteo's bad backhand) but Matteo is just a better and more experience player at this stage in their careers. This was purely a "tennis ability" loss, primarily because FAA's 1st serve is inconsistent and his forehand has been wild all season long. He should come away knowing exactly what he needs to work on to reach the next step.
He showed great mental fortitude in that Zverev match and the Cilic final 3 weeks ago was the best approach and resolve he's shown in a final yet. It's closer than you may think, then once he wins one, I think the floodgates are gonna open and you're gonna see him winning 250 and 500 titles at near Rublev-like rates.
Also fun note, when Berrettini was a month away from turning 21, he was ranked #270 in the world lol. Tons of time for FAA to develop still.