Fertilize my lemon tree?
In your climate you should be alright fertilizing now. As you know citrus trees love acid fertilizers. They are also prone to a condition caused chlorosis (as I remember from years ago) that is caused by an iron deficiency. It is characterized by yellow leaves with green veins. It is easy and inexpensive to correct with something like iron sulfate (but ask your nursery).
As for the insects, it is harder and harder for consumers to buy insecticides that actually do anything. And it gets expensive. I would call a pest control company. I think you will find it cheaper in the long run. They will know what to use and you can call them back if it doesn't work.
Boring insects can be fixed by giving them better reading materials. You should also look for scale insects on the bark on the underside of the leaves.
I envy you. I don't miss much about California but the dwarf citrus trees on my property. I had oranges almost all year round along with limes, lemons, grapefruit and tangerines. Unlike other fruit trees also on hand, you didn't have to race and deal with the citrus all at once.