Re your COAs. I’d go with No.1 and try to hang in there on the first few intervals on each VO2 session. I have found that the SSBII LV and Sustained Power Build LV has definitely increased my FU (quite a lot) but also my enabled me to complete the VO2 Max session at 100% (just!). If I were to focus on a VO2 dominated plan I’d use the Ramp Test to determine the FTP it is based on though.
As an aside for others…
For those who are reading this it links in with the thread I started below - but there are a few similar on the Forum now I’ve researched more. The first I’d heard of FU was when @stevemz replied to my post and it makes loads of sense and I really like Chad’s explanation on AACC Episode 189 at 31mins.