The good news: Amex tickets are sometimes cheaper that on the AA site, and yes you get 5x points if you use your Platinum card.
The bad news: Amex theoretically charges a fee (supposedly $39 but it might be going to $59) to touch a ticket, waived through December 31 but I'm not sure whether it will or won't be resumed on that date.
The ugly news: Amex's customer service is, at least in my experience, almost as bad as Comcast's, and that's saying something. I booked a number of flights in late summer and early fall and cancelled them due to the virus resurgence, and it was profoundly frustrating to rebook them. I usually keep pretty good records, but even I got lost in all the changes. For example, I had a ticket LAS-LAX-SFO in F that I paid something like $205.10 for and exchanged for a PHL-LAS tickets costing $79.01. When the ticket showed up, it showed a price of $205.10. I had them cancel that and rebook for $79.01, but I'm still not confident that I'll be able to actually use the $126.09 different.
The gold in the grave (the mcguffin in the movie The Good, The Bad, and The Ugly): I'm pretty sure Amex has made some mistakes in this process in my favor, and after all they've put me through plus the fact that I'm not sure they haven't made mistakes in their favor - I'm keeping the money.

AA charges $50 to take over an Amex ticket but they do it for free if you're EXP. I strongly suggest that if you are indeed EXP and you cancel an Amex ticket, call AA to get them to take it over before you cancel it. I have a lot more faith that Amex will accurately preserve the value of the ticket than that Amex will.