That’s awesome feedback, so glad you like Agenda so much, thank you for your support!
I hear you on the subreminders, but unfortunately as @michaelheinbockel already points out, this is the mess that Apple has created with the new reminders app.
I’ll try to keep my ranting to a minimum (you can get a taste here if you’re interested :D).
Basically, subreminders don’t exist outside the new reminders app, it’s something that the reminders app administrates itself, syncs independently of the actual reminders itself it seems, and doesn’t make available to the outside. The only bright side I guess is that subreminders are just normal reminders, which means that the subreminders do show up in Agenda (although we had to jump through hoops to make that happen due to yet another bug by Apple), but it’s not possible to show you the subreminders for a given top level reminder, which is I think what you’d like to see. The reminders app manages those relationships internally.
At the moment we have a bunch of open feedback items and bug reports with Apple on various ways they manage to screw up things, it’s obvious (and I guess logical) that under the time pressure of releasing the new features they forgot about the developer APIs. Most noticeable when it comes to handling URLs on reminders, but also with subreminders indeed. Here’s hoping that it will be corrected in iOS14/macOS 10.16 next summer.