Could you celebrate in the summertime (regardless of your actual anniversary date), have a casual ceremony in a park, then picnic/BBQ/potluck?
For the reasons you described, the actual excuse for the party isn't so important. Getting your loved ones together with a joyful focus is great (at some point, it seems you really only see people at weddings and funerals. A gathering where everyone is happy is wonderful, and with aged family members, you need to seize opportunities as they come up.)
Congrats on 30 years!