Just back from WDW. We had a blast watching fireworks from behind the castle, but over to the Fantasyland side. There's still so much to enjoy even without views of the castle. And you can simply show up to watch right when they start vs staking out a spot an hour early.
If you're out from your dinner in time, I'd make the quick walk behind the castle to the other side of it. The big fireworks shoot off from behind the back of Big Top Souvenirs. It's a bit loud back there, but fun to be able to walk and/or sit on a wall while enjoying the fireworks. One night DS and I rode Barnstormer over and over again while they went off - a memory we'll never forget!
Edited to add: It's certainly not as good as watching from the front. There are fireworks (and the projections) that come from the castle and there are the big fireworks coming from the back of the park. From the front of the castle it looks like a seamless production. From the rear you have to look back and forth a bit, in addition to losing the projections. BUT, for us, that was well worth it to get to keep enjoying the park until the moment the show started, plus the benefit of having plenty of room to move around and sit. So, if it's worth it to you to keep that ADR then you can still enjoy the show from a less desirable spot.