Is 34weeks too late to start gestational diabetes treatment? especially when traces of glucose in urine have been on for a while?

It's never too late. However, the fetus makes its own insulin starting in pregnancy and any excessive glucose passing into baby will eventually lead to retention of that excess in their tissues and organs. An oversized baby for dates is expected. Glucose control established well before delivery can reduce need for NICU care for glucose problems in the hours after delivery.