Very cool and interesting fish. Until this past year or so, I didn't even realize there were Osphronemus species aside from
goramy.
My old Osphronemus was also a garbage disposal, and I consciously tried to bulk up his diet with more vegetable protein and less animal. He never became fussy...about anything...and I think that is partially because I applied the same psychology to his feeding that my parents applied to my diet when I was a child. At every meal, without fail, I was always offered two choices: take it or leave it.
I think the slower growth, as long as it isn't carried to extremes, is likely a good thing rather than a bad. I'll bet if you overdo the animal protein...as I am certain I did when my O. was young...it would elicit faster-than-ideal growth.