Thanks for the pointers, much appreciated and I will think about whether to first push out the interval length, or to go for back to back days of 3x20.
On the balance point, I found this Jan 2022 post by Steve Neal on fasttalklabs about a regime which you just know would be massively effective
"The other with fitter athletes was to ride at what I call Lactate Balance Point or where muscle o2 sat would be just stable to slightly rising over long sessions over an hour, the athlete would hold this heart rate until power would decrease by 10% or they would reach 2 hours. I would repeat this for 3 days in a row, complete a recovery day, complete a strength day and coordination drills on the bike day, repeat.
With the above example, it is basically how I used to train athletes for about ten years, likely my most successful time as a coach. The program was a little dynamic in nature as some athletes could only do 2 days in a row and follow the rules while others could do 5. It was likely the best way to build an endurance/tempo base I have really ever tried, but it was monotonous. Although it worked I was worried and athletes becoming mentally stale (which never seemed to happen, but I was worried nonetheless."