I'm not a native English speaker, but this sounds strange to me. Is it grammatically correct to say "all the best forever" when wishing someone?
From your question, I think you're splitting up the sentence wrongly, because you are considering "all the best forever" as a separate part.
The correct split would be:
(I) (wish) (you all) (the best) (forever)
- (you all) = "all of you"
- (the best) = what I am wishing for
- (forever) = how long I want this to continue
However, in the interest of not sounding too heavy, I would drop the "forever".
I wish you all the best.
You don't need to say forever, because no one is particularly thinking that you are setting an expiration date on your wishes.
As per the comments, there is a second interpretation possible:
(I) (wish) (you) (all the best) (forever)
It doesn't particularly change the meaning of the sentence, only which part is stressed by the speaker. However, you can't determine that by reading it, it can only really be clear when spoken.