OP,

So on a typical ride you're doing 2k ft of climbing at low cadence in the big ring without your HR hitting Z3?

If that's correct, I'd definitely refer you back to gregf83's post, I think the first reply in the thread, and recommend you focus on training your threshold power, which is also essentially what Heathpack was saying about building hi-intensity cardio ability. You're not lacking leg strength, that's for sure.

You and I are the same age and weight, and spend about the same time riding both indoor and out, so perhaps my experience in training to get up hills is quite relevant. I did (and still do) train to build FTP, so max sustainable power, but faster climbing came from being able to maintain higher HR, higher PWR, and higher cadence.

I also have to pay particular effort to being able to return to moderate effort levels after punching up to max levels, because I'm really more of a diesel than a quick revving engine. I can get on top of a high Tempo power level and pump it out all day, but for me to respond with anaerobic power levels to an uphill attack, or to surge with that kind of power, really empties the tank in short order. I think a lot of it is just being comfortable working at high HR levels and being willing to suffer. I don't suffer well, I think, but knowing that my HR is going to settle down from near max, and that I will recover even at Tempo power, gives me the confidence drive harder.

That I do power training on the stationary has been great for me, because on the road, I've always tended to be conservative, afraid I'm going to bonk, and wanting to keep something in reserve for whatever may happen (will the group pace rise, for instance). On a stationary, I can perform exactly to the prescribed power level, and can drain the tank when I want without concern. Psychologically it's a real boon, but it pays out on the road in terms of performance, too; while I'm not one of the fastest climbers on Strava segments overall, maybe top 15% of a few hundred riders on average, but in the 200lb+ category, I'm usually vying for first.

You should probably reconsider investing in a power-enabled trainer or consider a bike meter of some type, so that you can use power to really dial in on your goal. Power makes training to plan quite easy.