Have you tried pacing with HR? I did it on the Poor Student on Sat. I set alarms at 120 bpm and 140 bpm. At 120 bpm I’d up the effort and 140 bpm I’d back off. Got round an hour faster than the Santa Special two weeks before. This is despite having 1.5 times the climbing of the latter. I put this down to not burning my matches early on, using more fat for fuel, leading to no second half fade. I also felt great at the end of PS compared to a bit wrecked at end of Santa Special.
The most illuminating thing was how hard I was working when starting audaxes, and no wonder I faded later on and didn’t feel quite so great after a few hours.
Hadn’t worn a chest strap on audax for 7 years, but plan to do so for next three 200’s. In that time I’ll learn to associate with perceived exertion, ready for the longer distances, as my main navigation GPS doesn’t do HR and my old edge 500 that does only has a 15 hour battery life.
Obviously what you set the alarms at depends on whether you have a hamster heart or that of an Ox.