Have you tried acidic irrigation water? This seems to work a lot better than acid fertilizers or acid mediums. That means using rainwater or adding some vinegar (the amount depends on your tap water pH and alkalinity that you can find out from your public works department) at every watering.
What's the burrowing insect problem you're having? I've had blossom midge devastating my hibiscus and Sambac jasmine for a long time now
There are control options, but I hate spraying things unless I have no other choice.