^that's what i'd do too...starve and feed. they aren't picky. i think trying softer sinking pellets would work better than harder ones (hikari massivores are generally soft, but omega one pellets are hard).
i had my endli for 2 months and he ate the whole shoal 6 tiger barbs and countless other tank mates feed blood worms and beef heart shrimp then one day i saw him eat a massivore...ow he'll eat 6 at a time...but just try and wait fish take time my was a accident but i was cheering after he did it lol try starvign him for a bit