I have a female betta around that size I picked up from my LFS. I just feed her frozen brine shrimp/bloodworms and NLS small fish formula pellets. She has no issues eating any of these foods and she will even take the large 1mm pellets I feed the others.
At that age, you could really wean off BBS. At least live BBS. I doubt that's what it was getting fed in the store/wholesaler. My wild fry are smaller at two months old, and I only give them BBS as a treat. The rest of the time it's grindal worms or pellets.
I hate hatching BBS so it could colour my opinion a bit haha. But I have seen spectacular growth on my fry with just the grindals and NLS .5mm pellets.