Ive not read the whole thread but how do you know 204bpm isn’t your max?
Iam 45 and the maximum I’ve hit this year 199bpm (96-98% of that is regular on turbore sessions and TTs on regular rides though I only hit a circa comfortable 170bpm) and my mate (the same age as me) regularly hits 200bpm +
Conversely though when I was about 30 my younger mate had a max HR of 170 something and he’s a phenomenal climber. We are all different and formula calculated methods can be way out.