Updating this thread.
Around the first of the year, sensing I wasn't going to accomplish anything by continuing to ride, I took a full month off, tried to ride once, took another month off, rode once or twice, and took another three or four weeks off. By the first of April, I had only gone 100 miles all year. In the latter part of this period, I was suddenly overcome with extreme fatigue and tiredness, which inspired me to find a good doctor to run a complete panel and start really getting to the bottom of things.
Everything in the bloodwork was normal with the exception of glucose (just 1 MG/DL high) and white blood cell count (3.8; 4.1-10.9 is normal range). My cortisol has been tested twice seven months apart (was 21 via an ACTH stimulation test, 18 via blood draw, both in range). Testosterone has climbed from 308 to 389 to now 534, so that's a positive. EKG with a cardiologist was normal. It was noted that my heart rate climbs about 35-50 bpm upon standing and blood pressure drops. HR drops back to around 75, blood pressure stays lower. Cardiologist noted that a hormone imbalance can make heart rate not only higher, but erratic.
At this stage, it still feels like my body has a governor on it when I attempt to ride. I extended my time to 2 hours on one occasion at a very slow pace on flat road, but there's just no "go" and anything beyond a very leisurely pace and there's the feeling off lactic acid buildup/soreness the way an all-out effort would have in years past. Still haven't suffered from insomnia, nor have I experienced any infections or illnesses.
Right now, to improve mood and perhaps boost testosterone and improve immune function, I'm riding 3-4 times a week for an hour at a very slow pace, but there's been no noticeable turn-around in performance.