Since I discovered intervals.icu, I have been trying to create more and more tailormade training sessions and plans, and the more I learn and read about the subject, the more questions I have:
How do I know which intensity I should be able to hold with how much rest in between. How many reps am I supposed to be able to do? Should I be able to do 3x 10 min reps at my ftp? or 4x? 5x? How do I know when I am doing too much or too little? Also how to I adjust target depending on resting time? for instance 3 minutes intervals can be done with lower intensity and really short rest to work on repetitive efforts, or with 10 min rests and really high intensity during intervals to completely recover between - and both wil work quite different aspects.
I have learned that one of the best principle for progression is progressivity, with increasing intervals lengths, or reducing rest time reather than looking to increase power, but by how much should I do that?
I know that’s quite a lot of questions, and probably getting a coach would be the easiest option. But I am also quite interested in all those topics so don’t mind reading books or do some training on the subject. So if you have any resources you can share on the subject, I would be quite happy to see those 
Or maybe I should be more looking into some kind of repostory of base sessions that work specific aspects and how to make them evolve week on week in order to progress?
Thanks!
PS: just to give a bit of background, I am training 20h/ week most weeks, running and cycling, doing quite a lot of 1/1 interval/rest VO2Max sessions, threshold, tempo, endurance, (trying to) respect the 80/20 ratio of intensities, However, I feel I have kind of reached a plateau now but could still progress with much more specific blocks of training to work on my weaknesses.