You seem to have figured it out yourself: your low consistency last season gave you an artificially high estimate of your then-current ability because you got a lot more rest than you were supposed to, then the “sizable jump in FTP going to build” made every unit of TSS much harder work than it used to be. You got stronger, but you didn’t get much better condition, and now it’s coming back to bite you.
I recently went through a similar thought process (see my thread on choosing between Base and Build for all the gory details), but my conclusion would seem to apply to you: you’re failing due to fatigue, and backing down to Low Volume would be the best choice. Once you’re comfortable and thriving on Low Volume, you can always add a ride or add intensity… I’m doing LV plus a weekly long outside ride, and loving it, whereas MV was killing me.