This is probably highly personal but just in case this small data point is useful.
I've been consistently training and improving my aerobic capacity and doing strength as well. As a 32yo man, the latest AeT result was around 180. I was actually not sure about the number because it seemed too high, but anyway that session was using consistent intensity, with purely nasal breath. The main form of training is climbing an inclined treadmill. On real trails, I did feel easier, e.g. I once hiked on a peak from ~3000m up to ~4000m, with HR at 160, nose breathed all the way without rest.
Now I am in Nepal, ebc area. The HR with which I could nose breathe is much lower. I climbed lobuche east, 5000m - 6000m, and when my HR touched 165-175, I had to breathe very hard: no buff, push hard to inhale and exhale, usually using the mouth. Even at lower intensities, I often couldn't keep the buff on, or nose breathe. Instead I would only cover my mouth, use the nose to inhale and mouth to exhale. Breathing has become a limiting factor. I also guess that I became more reliant on sugar, because even after a big breakfast I still had that feeling of low energy on the way. I did drink a small amount of coke cola on the way up. Acclimatization was good, as I didn't have any symptoms like headache or lack of appetite. No DOMS afterwards at all.
Basically at this higher altitude, I have a strong feeling of being untrained. Not sure if it's just a lack of general fitness, or suboptimal acclimatization or what. I'm adding energy gels to my toolkit because energy chews helped a lot before I had any formal training, and gels might be easier to use at altitude.