You have been overfeeding your betta. Betta's stomach is as small as its eye. You only need to feed once a day instead of two meals. How big are the granules? Even for 0.5mm pellets, 3~5 of them is more than enough for a betta. If it is 1mm, then 1~3 should be enough. Remember, once a day, not twice a day.
Filter must be turned on 24/7. If the filter is too strong for the tank, then you are using the wrong filter. I highly recommend to use a sponge filter for betta tank. Since they do not like current.
New fish typically need time to adjust to their new environment. It is normal that they are not very active at first. By adding more decoration and plants to create hiding places can make them feel more comfortable and thus become more active.
