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From my understanding, you can do endurance and sweet spot during a fast, although sweet spot can feel really tough when your glycogen stores are depleted. I would do it at the end of your fast though and eat afterwards, not eating after is where you will likely do damage.
I did intermittent fasting (16-8) whilst doing a Trainerroad low volume plan, but tried to do my workouts in the afternoon, 3-4hrs after my lunch. If you are actually trying to build your ftp then properly fuelled workouts will be much more effective.
Start with 16/8 and see how you’re feeling. I’ve been doing it for around 4 years now for various health/longevity reasons and use 16/8 as a baseline. Often ends up being 18/6 or 20/4.