Flyertalk was right: you can still use the United Travelbank “hack” and receive the $200 yearly American Express Platinum airline reimbursement.
As a data point, I made the purchases two minutes apart and AmEx didn’t care. but I’m guessing it’s best to keep the amounts in multiples of $50.
Counting the reimbursement, the annual fee drops from $450 to $250, which is still expensive, but the numerous benefits I receive make this card extremely valuable and, if the internet is true and there are offers of 100,000 Membership Rewards points sign-up bonuses, you should consider applying. A sign-up bonus of 75,000 MR points would be worth it, too.