Beat this high score for every single one - you'll know when you've done it at the end because the Flan will jump up and down, clapping it's short arms - and when you've done all seven, the final Orichalcum+ will be yours. The earliest you can get it by is when you get to The Final world so it'll be just before all the final bosses.
Can confirm, it can be acquired before the end of the game. Every Flan mini game has a score threshold that you need to beat to earn three of the respective ingredient, and a new ability. AFAIK all of these can be obtained before fighting the final bosses.
You need to participate in each of their mini games to earn a trophy, as explained in our Kingdom Hearts 3 Flanmeister award guide, but you need to do slightly more than that for the Orichalcum+. As you complete each of the main worlds in Kingdom Hearts 3, there will be a Flan waiting for you somewhere when you return. This guide will tell you everything you need to know to synthesize the Ultima Weapon, it is a 100 synthesis guide with all material locations and also tells.
