The 20m last ball shot would be better if it was toned down to something like the 5m you suggest. Although the 20m isn't completely off base since you can score 5m any ball from the fish bonus kick out if the 10x is lit.
To me the jackpot can get out of hand if it hasn't been collected in a while. It seems kinda easy if you light it as a seafood wheel award and then score it.
I still really enjoy this game despite a few scoring balance issues.