I don't think you can see fireworks inside restaurant. The best position is in front of castle.
But 1 thing, last time, I spent 2 days there but there is no fireworks. The excuse is the bad weather but that's not truth. I believe if the no. of visitors are not enough, they will cancel.
main street have a one restaurant called "sweet heart" cafe near castle can see 煙花 but need to sit in outside seat not inside seat and need to go restaurant before 1/2 hours of 放煙花 time in order to getting the seat first.
enjoy!