Currently using it to chop rice to straw. The knife, despite saying it is unbreakable, is very much taking damage.

That's unfortunately not something I can fix on my end. Farmer's Delight must not be checking whether the knife used to chop on the cutting board isDamageable, before applying damage to the knife.

Are you not using the standard unbreakable component? If I give myself a knife with that, it works fine and never breaks on the cutting board:

/give MuteTiefling aquaculture:neptunium_fillet_knife[minecraft:unbreakable={show_in_tooltip:true}]

Excellent call with the vanilla "Unreakable" component. Wasn't a thing when the Neptunium Fillet Knife was originally designed.
I'll release a fix shortly