Wdobrow, this isn’t my area of particular knowledge, but one thing jumps out at me right away from the training schedule you outlined: I don’t see any recovery weeks. The typical training cycle is three weeks build, followed by one week of recovery at 50%-75% volume of the last build week. In TftUA, a 50% reduction in volume is recommended for the transition period. For your week four, that would be 2.5 hours. The recovery period is when your body adapts to the training stress you have put it under. This is when the gains occur.
How much recovery your body needs is of course very individual, and is influenced also by the other stressors in your life (work, family, travel, etc.) and also age. For example, I have found my body responds best to a 3-week training cycle: two weeks build followed by a recovery week.
It does sound like you’re probably overtrained. If it were my body, I’d back off, make sure I was eating and sleeping well, do a little light activity (e.g. a walk) if it feels good, but if the body says “meh” when I suggest a walk, then I take that as a no. Swimming in a relaxed way can feel really good.
This article explains the physiological and other indicators of overtraining (it sounds like you might have already read it if you’re doing the stairs test, but here it is anyway): https://uphillathlete.com/overtraining/
This article gives tips to aid recovery: https://uphillathlete.com/tips-to-aid-recovery/