I was over-thinking it as usual instead of being simply bull-headed about it. Doing the latter, I can now ride for 2 hours at 75% FTP, HR at 84% LTHR, no drift. That was my goal for the end of the year, doing that several times/week. Done. Now I've added one interval day/week, 105% FTP. Just did a set of 4 X 3' X 3' to get a feel for it. Went just over LTHR, almost reaching VT2 on the last one, so I'm in the ballpark. Next week I'll try 4 X 4' X 2' and more 120' at 75%.
References I've looked at lately:
https://www.researchgate.net/publica...trolled_Trials
https://elementssystem.com/wp-conten.../05/Hydren.pdf
https://www.researchgate.net/publica...ained_cyclists