Betta Fish: What’s a normal weight gain timeline?

I got Finny from PetCo in early September, and he’s been doing SO great since coming home! His fins have started regrowing and changing color, he’s active and inquisitive, and overall seems very happy.

However, he still seems skinnier from the top view than the ideal body condition on the betta body condition chart. Is a month and a half too soon to see much improvement, or should I worry he’s got internal parasites/change up my feeding regimen?

He’s fed twice a day and gets either frozen baby brine shrimp (around 6 or so at a time?) or frozen daphnia (a couple tiny droppers worth) alternating feedings, with the occasional Omega Betta pellets (only like 2 times a week, and gets about 4-5 small pellets at a time in the span of like 5 minutes).

Obligatory tank info:
20 gal long with sponge filter and heater set to 78
Ammonia: 0ppm
Nitrites: 0ppm
Nitrates: 20ppm