Is there anyway to keep glass from splintering when you drill it? I had it sandwiched pretty tight between two blocks of wood and I still got a fair bit of splintering.
I keep tape on it and like you , drill with wood backer clamped on and get along pretty well . Slow n easy...
Then I use a small about 1/2" grinder ball on holes to smooth up. It is just an abrasive stone type.
X2 on what Kenny said. I will add you could put overlays on the limbs before you drill.
Use sharp bits tape and backer board dont rush it.