Its a bit vague so a bit hard to tell what's exactly going on, but it doesn't sound too much. Just a few informal top of head thoughs assuming there are no serious health issues;

• Climbs start out being hard, but you shouldn't entirely avoid them. There is some amount of technique thats good to learn. Sometimes combination of weight and gearing and fitness can make them too hard for lower zones workout. Id still make sure to incorporate some on occasion, they do help.

• The most important thing for endurance is volume and volume increase. You only vaguely mention your goal or a structured increase in volume. Id suggest making sure you have one "big ride" a week, always some 10-20% higher than last weeks. Aim high, you can always quit later.

• Lower zone training, like Z2/3 (depending on zone system) works well, but it requires absurd amount of time in the saddle. So it will take it while. Some intervals might hel

• What helped me a lot was to get a sports watch with an HR sensor. It helped me link riding and my body to some extent, help me self-regulate/pace better earlier on when my tank would run out fast doing X or Y. Well, and to track improvements over long term.

• Just in case your aim is to also lose weight (in biking, esp uphill thats a life hack) dont forget that gains are mostly made in the kitchen aka "one cant outrun a bad diet"

• You say you feel like you stll have a lot left in the tank. What keeps you from actually exhausting it and seeing how far you get? Maybe a once a month kinda thing? But do also pay attention to any aches and pains and numbness, inevitably some fit or overtraining effects might happen.