J'aime bien déguster le chocolat noir parce que d'abord, il est bon pourà la santé.

Il contient beaucoup d'antioxiydants et d'autres éléments qui peuvent améliorer la/les performance(s) du cerveau.

"antioxydant" is considered countable, so you may pluralize it
Likewise, it's usual to pluralize "performance" when there are several domains/fields something can be performant in, but the singular isn't wrong at all

De plus, le chocolat noir ne contient qu'une petite partiefaible quantité de sucre, ce qui lui rende d’êtrefait de lui une bonne alternative pour couper le coupe-faim (entre les repas que les), comparé aux biscuits, les glaces... etc.

It's also a noun ("coupe-faim") that can also be used as an adjective
You can add "entre les repas" but you don't need it

C'est comme le café, on peut sentir les goûts amer, aigâpre et sucré, ce qui sont devformenut une synchrharmonie parfaite en bouche.

"aigre" is rather acid (酸), like pomelos, some unripe fruits, lemons and so on
"âpre" is like truffles, some vegetables or sugarless cacao
"synchronie" is the state of doing one thing in the same time; "harmonie" is the idea of several things (usually sounds, as it's a music-related term, originally) forming one and producing something pleasurable. It's a bit like 諧 and its components.