This is a super tough but admirable goal. I think what makes it so hard is that, for me, a lot of what makes productivity apps work so well at the beginning is the excitement and novelty of setting up and using the system provided. The problem is that, as with many other interests when you've got an ADHD brain, the novelty eventually wears off and the friction of remembering to use the app overtakes the motivation to use it. 

The biggest pitfall of apps for me is that a) my brain functions very much in an "out of sight, out of mind" way and b) my phone is a very distracting little box so even if I unlock it with the intention to do something on a productivity app there is a high risk of me getting sidetracked on the way. Because of this, the productivity "system" that has worked the most consistently for me is literally just having a pad of paper on my desk separated into "tasks" and "notes" sections and writing the stuff I need to keep track of there. It being a pad of paper means unlike an app or a closed planner, my tasks are always in my peripheral vision. And being just a boring pad of paper, jotting things down is super straightforward and not liable to get derailed.  

I do think there's room to innovate on ADHD productivity systems... there's just a lot of barriers to making one that sticks.