Muscle oxygen sensor strength training
This article features a relatively new method for determining your aerobic threshold (AT) heart rate using a kind of nonlinear HRV analysis: DFA alpha 1. AT correlates with a DFA a1 of 0.75.
This can be done with a relatively inexpensive HR monitor, such as the Polar H9, and a free (Kubios) or inexpensive (HRV Logger) app on your phone. No need for expensive physio lab testing! And you can repeat as often as you like once you've purchased what's needed.
For more information, you can do a web search for "dfa a1 aerobic threshold".
Many articles I've found say that ME/CFS patients can estimate their AT by calculating 60% of their max HR, but this method may give us a more accurate number. Please note, this measure was developed using healthy participants, so it may not translate for us.