Ha, then you are clearly more experienced than I am and I can see why you would want to try something else if working upper Z2 your progress stalled. I would probably still want to tweak SSBHV a little. I would prefer longer intervals and just progress time in zone. Also, it still seems too much SS (5 times a week!). I would at least consider replacing some with continuous tempo (think Polar Bear). In any case, please report back how it went, I am interested to know.