True, you can only make BOG reservations for dinner, not lunch.
However, some people have been able to do BOG lunch FastPass+ about 30 days before their trip. That would certainly cut down any wait time.
If your dessert party is at 8, I think a 6p dinner at BOG is cutting it close. If dessert party is at 9, then you should have time but might be too full.
If you can get a dinner reservation, why not? And then if you can get a lunch FastPass+, you can always cancel the dinner.
Hopefully, you'll get in to BOG on your trip, one way or another. Good luck!