//snip

Quoting your age in a reference to Zone 4 heart rate suggests to me that you don't know what zone you are actually in.
When was the last time you measured your MAX bpm?

There is an established relationship between heart rate and age, but it is imprecise. Toss the formulas out the window. My own MAX is 192 (for this year) and I'm 57 y.o.
Your MAX could be much higher than you think. Check it.

I agree with the rest who suggested higher rpm @ lower gear. Aim to maintain at least 90, and that includes while ascending.