Hi Robert – there are numerous reasons, why some athletes struggle at the end of endurance events. You seem to have tried a lot of different things, but did you change things, too often and too drastically? It’s imperative, to stick to a new approach, for an extended duration, and go through the FULL process, before micro adjusting.
How you mix your training, according to your pedigree, circumstances and goals, is the key to long-term improvements. Too many athletes, like to try blocks of “extremes” which eventually leads to diminishing returns, instead of modulating the intensity, withing the program.
It seems you might be overthinking it, with the VO2 Max and heart rate stuff. Running is such a primal sport, possibly the most primal of them all. Focusing on your instincts, and sensations will allow you to get the most out of yourself.
In the end, a lot of athletes do some things improperly, without knowing it. One reason, why even top athletes, in all sports, have coaches, is that they provide an exterior and objective outlook!