Agreed. Going the Dade route instead of Spencer (mentioned by @mcalista) is a good option. Another one to consider is ignoring the descending parts of Mills.
The workout instructions and text in Mills talk a lot about finding the right power level for VO2 intervals, emphasising that it won’t be 120% for everyone. However, I feel like the descending ramps undermine this (and also undermine the progression to Spencer). The two minutes passes so much more quickly with the descent there, and then makes 3 minutes the next week even harder to get through. So you can try to do Mills out of ERG mode, ignore the descents, and see what % of FTP you settle on as repeatable for the full 2-minute duration. Whichever number you settle on, use that to adjust the intensity of next week’s Spencer.