I would reccomend getting frozen bloodworms instead of freezedried. You can alternate the brine and bloodworms, and then now and then try the pellets. Sometimes they decide to like other foods after eating a variety
well other than the fact that frozen food will chew a hole in your wallet there sin't anything too bad. However i feed those kind of food as a "treat" to my bettas rather than the main meal.
I hav ea betta who hates hikari bio gold, but loves the HBH betta bites, you might wnat to try using another type of pellets. You could also try to take the pellet and soak it in the water from the frozen brine shrimp and feed it to them.
That might give the pellet some taste of the brien shrimp and the betta might eat it. Maybe try feeding pelletse while your feeding the brine shrimp, he might accidently eat a few pellets and realize it's not that bad after all