So since you're doing multiple nights, I would divide and conquer: Disneyland night shows one night, DCA night shows on another. If you're willing to stay late, you're going to have better viewing each time.
Basically, this is how I would do it:
Disneyland Night: Watch Fireworks from as close to the castle as possible nearer to the Frontierland entrance. As soon as Fireworks are over (the Christmas ones are much shorter than the regular shows) I would immediately head in to watch the SECOND Fantasmic. And yes, I would get a fastpass for it. The advantage is that the 10:30pm showing is always less crowded than the first showing and a lot of people are done for the day at that point and are all exiting the park, which means you'll be headed in the opposite direction of the crowds. Also there are more space options since you have to wait for one crowd to leave before another one can take it's place.
I recommend this strategy because frankly I don't think fireworks looks as great without the actual castle in view and if you watch the first Fantasmic you're pretty much going to see them from Rivers of America because 1) the start fireworks immediately after the show and 2) it's crowded from the show.
DCA Night: Same strategy. Try to catch the earlier Paint the Night (assuming they still have it, it's a big assumption) and then watch the show from close to the WOC viewing area. I would probably recommend somewhere near Pacific Wharf entrance all the way down to Ariel's Undersea adventure. WOC is ridiculously popular, especially during the Christmas season, and since both showings are a lot earlier, it will be more crowded, even the second showing, so be prepared to arrive at least 30-45 minutes before the show.
I don't think you need to book a dinner package for anything and definitely do your research to find out where exactly has the best views since there are a lot of articles on this already.
Also a note: if you are fastpassing super early in the day, MAKE SURE ALL YOUR TICKETS HAVE BEEN SCANNED FOR THE PARKS BEFORE YOU TRY TO GET A SHOW FASTPASS. I can't tell you how many times I've had a dad rushing up to me at 8am trying to get WOC fastpasses for his family who were still at the hotel only to find out he would have to come back AFTER all their tickets have been scanned. Fastpass cannot work unless your ticket number has been activated for that day, so just a big FYI out there.
And Dec 15-18 might not be peak busy season, but the parks will still be packed. So be prepared for that.