I've done this one twice with JY, and may do it a third time in due course! Lake Garda is really wonderful, it is SO big that it can deal with the many tourists who flock there. The road around the lake is slow, but on a coach with new friends and a knowledgeable guide, it's not a problem. Limone is my favourite little town, magical views, wonderful coffee shops, great place for gelato.
I'm hardly an opera buff, but I loved every second of the Aida we saw past Summer. I paid for the top upgrade and thought my comfy seat was well worth it. If you go for the JY standard seats, you get a better panoramic view than in the posh seats, but do buy/hire a cushion or two!!
The group makes the holiday and although one can get an occasional irritant, 99% of people are great company. You meet lovely people with JY. Remember that many people on any JY holiday are as nervous as you! I really hope you enjoy Lake Garda and the opera!