re: adding volume
The conversation started with adding more Z1 and Z2 rides and then turned to not inflaming the sympathetic nervous system (by adding more volume) and trying to focus on parasympathetic nervous system boosting (non-cycling) activities.
I believe it was Seiler (et al) who found that “highly trained subjects have rapid autonomic nervous system recovery after long, slow distance sessions” and “exercise below the first ventilatory threshold causes minimal disturbance in ANS balance”.
I can only assume the question asker (Ivan? Iver?) is in the ball park of “highly trained” what with doing 800-1000 TSS/wk. With that in mind, he should be able to add, as was suggested, more <Z2 volume without much physiological negative disturbance. Not that there’s anything wrong with PSN activity.
From an anecdotal standpoint, last summer I did 4 months of high volume only-Z2/<VT1 riding. At the beginning, my SNS and PNS were scarily out of whack (taken from HRV data). The only two things I did differently were transitioning to a high fat/lower carb diet (but not strict HFLC) and doing no intensity over VT1. Over that 4 month period, my LF/HF ratio went from 6.8 down to 0.46 (a sign of PNS dominance); HF (a PSN proxy) went from 500 to 5,000 (low is bad, high is good). During this same period I was also incredibly mentally stressed out – not PSN friendly! – yet simple non-intensive exercise (a lot of it) reduced an out of control SNS and revitalized a damaged PSN with great and lasting effect.
Adding more low intensity rides may also be more physiologically beneficial to the questioning rider than starting a new PSN-centric activity like yoga or meditation where there could be a long and/or steep learning curve (e.g. frustration, high cognitive load, yet another new routine, etc.). Getting on the bike is, as said in the podcast, Pavlovian; he knows what to do and can most likely do Z2 rides without much mental engagement/stress (PSN friendly).
My vote is for the bike, two birds and all that. But, also a perfect time to try out all those new and/or neglected things. 