Thanks for this update, Brett. Tried Exist a while ago and am excited to revisit it.
If it is customization you are after, though, I am huge fan of Reporter, which pings you at intervals of your choice to ask you a series of questions of your choosing. It's infinitely customizable, which is great, but the trade-off is that it isn't cloud based, so no pulling in data from other APIs or using AI for correlations.
That said, it automatically pulls in steps, location, and even ambient audio level from your phone's sensors.
I've been using it to track my mood on a 1-7 scale similar to Moodnotes, as well as tagging where I am, who I am with, and what I am doing. Now, while it has some visualization and filtering, you can gain some interesting insight from dumping the CSV export into Vizable for iOS. In my case, I can see who and what makes me grumpy, how that correlates to my step count, what I'm working on, etc.
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