In my experience, the higher I get, the lower my HR goes. So, I would mainly concentrate on Z1/2 speeds. The climbing should be not so technical, right? That should be doable with climbing gym training. No clue if climbing with a weight vest or a backpack makes sense in general. Happy to hear about that from others.
I would concentrate on creating endurance by using a graded treadmill (if possible for you), cycling or ski rollers and adding gym ME for steepness.
Try to maintain your existing ME level from summer and spring with a maintenance gym ME workout.
Let us know what you did.