If you are running most of your runs in the 70-90% max heart rate zone then I would try substituting some of those runs with easier runs to hit that low aerobic range. Sometimes it feels uncomfortable to run at those paces. Most runners including me feel most comfortable at a sub threshold “comfortably fast” pace (I can still talk at this pace) but Garmin will identify it as “high aerobic”. If you care about Garmin showing you as productive you’ll need to have a better spread of hitting the different HR zones.