Advanced reminders are going to be a thing for the next big release. I agree that one problem is to pass the unlock, but staying on track with your intention is a different story. One periodical notification with static text can in theory fix that, but the chances are low in comparison to the full-screen pop-up. I intentionally focused on the unlock procedure first. For now, you can combine it with other apps like minimalistic launchers and apps that pop up after the app opens. But eventually, improving the reminder experience can make the solution more complete, I agree.
About typing "captcha" or random characters. I think it's just a different type of nudge. Another can be a small mini-game to play like catching a moving object. I'm going to consider adding different types of nudges to the app. Thanks for the suggestion.
One thing to consider, maybe open the app up for interaction with Tasker (being able to send events and provide actions to execute) - this will allow people to implement advanced logic on their own. If you expose user answering or skipping the screen as event, and ability to bring the screen back up as action, users[0] would be able to easily add features like "bring up Intenty screen when user attempts to open specific app or apps during work hours", or "make skipping require solving an ordinary differential equation shown rot13-encoded, and write down the answer in Klingon", or whatnot. That could be a good testbed for ideas to later incorporate into the app itself.
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[0] - Those that also use Tasker. I'd wager that for your target audience, the proportion of such users is much higher than in average case.
Just thinking about this, there might be room for a home launcher that helps manage attention this way. Probably more work than you are thinking of doing though.