I think the entire theory about the intervals is overcomplicated and too much of an overgeneralization at the same time. If users could just have buttons with "110% of last interval", "+2 days", "1 week" that would be the best way. I've written a web app like that many years ago but js development has since outpaced my motivation to keep up.

Steps is just manual entry of the first intervals, right? That's very static.