From my perspective it looks like are doing several things right. I am not sure that spending focused time in zone 3 is going be helpful as that tends to be not hard enough to be a great training stimulus and too hard to be a recovery workout. I would suggest dropping the intensity there and increasing the intensity on your interval day to get more in the 4-4+ zone.
As far as the muscular endurance, you should check out the article in training about muscular endurance and see how it makes sense to apply it (https://uphillathlete.com/vertical-beast-mode-what-is-muscular-endurance-why-it-is-important-for-any-alpinist-or-mountaineer-and-how-do-you-train-it/) Part of what you are trying to accomplish may be met with training on heavier equipment, but the load is probably not enough to fully reach your potential.
Good luck in your racing and adventures,