I'm not sure what your dinner reservation is for but I would allow plenty of "stuffing your face time" trust me, you'll want that! lol!
For your 830 option, I just don't think that is doable for both.
Allow for a full hour for dinner and some stroll time or loop time to get you from TE outpost to the sing along. How will you be getting there? Golf cart? or bus? or bike? You can get from the outpost to the sing a long in about 5 minutes by cart, 15 by bus depending on wait time, 5 -10 by bike and 15 to walk it.
Now they light the fire at 7pm. Do you plan on doing smores before the sing a long? If so, 7 should be your arrival time. Sing a long starts at 730 and chip n dale come out around 750 - 800 pm. Do you plan to stay for chip n dale? It taken a good 30 minutes for those little squirrels to make their way through the crowd. So if you plan to stay for them, then I say no you don't have enough time. You have to get yourself back to the outpost before 830 (by TE).
Now if it were me, I would be checking my clock every 5 minutes and not able to really enjoy the show plus have to leave early from the sing a long to make it. No way by bus or cart or walk.
So I would either skip the sing a long or skip the 830 wagon ride.
How here would be the best option...6pm Wagon Ride!
TE early dinner 4:45 or 5:00 pm, then catch the 6 pm wagon ride which is literally 20 steps away from the TE porch.
Do the wagon ride and make the fire at 7pm, sing a long 730 and stay for the meet in greet.
Get a few beverages and find a spot along the beach before the 9pm fireworks and hang out and enjoy the light parade that follows.
Have a great trip!