Gman, you are sort-of correct about marbling. To me, raw beef fat doesn't taste good at all. When it melts it contributes flavourful gelatinous moistness to the meat or anything it touches. That's why McDonalds only used beet fat in their fryers for decades.
Most Americans don't realize that Florida produces the most beef in the USA. Their grass grows year round and as you noticed makes the best cattle feed for flavour, in my opinion. Grass fed beef is high in both Omega-3 and 6 fatty acids which contribute to wellness in humans. The feed lot corn fed beef has very little Omega-3 and less flavour.
Prime grade beef isn't necessarily better, just fattier. To serve my guests, i always bought well aged Top-Choice grade beef.
That Picanha cut isn't just Brazilian, it's their name for Top Sirloin Cap. That butcher might have been confused. If he sells sirloin steak or roasts every sub-primal Sirloin pack he gets contains the Picanha. It's right in the middle on the outside. "Whole butt" isn't a beef part i know.
Burning never works.
For your own burger mix, try a blend of chuck and sirloin.
Form your patties, vacuum package according to use, SV cook to Pasteurize, chill and hold. When you need a burger, warm <130F, season and grill, or however you cook them with or without the mayo.