At 3 to 4 rides a week and that amount of mileage you should feel like superman on a bike! I can only wish I could make time to ride that much mileage.

If you are keeping your HR low in zone2. Then I'd think you aren't doing anything that might increase your VO2max on a bicycle. You'll need to do some intervals or HIIT. For me the rolling terrain here essentially has me doing intervals. And most of my rides at a fair amount of intensity. On solo rides, I can't get much z2 hr at all. The majority of my time alternates between z3 and z4 HR. (based on lactate threshold). The overwhelming majority of my rides are solo since my son got too busy with his business to ride with me.

As I mentioned earlier, make sure your personal data and heart rate information is accurate. Like Garmin, Zwift is just making a guess based on certain metrics it sees for you. Not the information that a true VO2max test in a lab will rely on. Which involves a mask to be worn to measure the O2 in and CO2 being expelled as you run a treadmill or pedal a trainer.

I don't really train either. Not in a formal and planned way. Other than I try to do the things that will get me a new PB (personal best) on certain segments I track. The hills here, unlike the flat land where I use to live, don't make it easy to do simply a relaxing slow ride. At least not solo.