FYI, Concerning Pillar and training zones in general, I did figure out that my Garmin Edge 530 does allow me to display time in zones and of course to define those zone values. I made a page for this. This should make it easier to make sure I get the proper time in zones, at least for endurance type sessions. Obviously intervals are going to be what they are.
Also, concerning Pillar, I found some inconsistencies in some values that are confusing. For example, I found where it calculated my power zones and for example my zone 1 is 102-129 W. Yet in today's zone 1 workout, the description was zone 1 (118-129 W). Yet I analyzed the data from the ride and the reported time in zone 1 matched up with the time spent in the range of 102-129 W (the defined zone). So, this may just be some bug in how they display the workout description.
But there were several other things that made no sense in the "Session Accuracy Breakdown". I'm not going to go into detail here just yet. I messaged Pillar about this and I'll see what they have to say. To me it looks like there may be some bugs in how they compute values but also just confusion between the target of the sessions and what I actually did. I'm hoping this is at most just some user interface issues and not reflective of what they are doing under the hood to compute the training schedule.
I'm hoping Pillar will work out, as it sure seems to be just the type of thing I wanted. Which is to let me define a goal, figure out a training schedule, but give me some flexibility in when I can ride and for how long. Early days, but I have my hopes.