Where I live I’m either climbing grades in the 10-16% range or descending at 35-40 mph. Only about 25-30% of a typical ride is on the flats. If you don’t like climbing, you have to go elsewhere.
Zwift is a nice training tool, but it’s not the real world. Get outside and ride. The areas outside of Boston have great hills.
To become a better climber you should consider cross-training. Make time a few days a week to get to a gym and lift weights. Also, in my opinion a high VO2 max is more important than pure power. Find a complementary activity, such as running, to increase your VO2 max. I’m 58 with a VO2 max in the high 40’s. I attribute this to 30+ years of cross-training and always seeking a new goal.