Is it safe to keep using the board as it is, or should I repair it before foiling again?

No that's cactus and need box replaced. Not an uncommon issue but you can't use it like that it will tear thru.
as an interim you could make a deck plate and use two longer bolts to deck
And stop shimming the head if you have been - that's what loads the bolts instead of it jamming in the tapers. Tighten, rock it back in forth tighten again - repeat. If it's not sitting on the front and rear tapers it's not fitted right and the bolts wreck the box just like that