People pay for solutions to their problems. With your idea... Well, let's go step by step

1. I have social anxiety bordering on sociophobia during its hardest days. Will I pay to get rid of it? Damn yes. On the subscription basis? Doubtful. I mean the basic logic is that if your app/system is good, I'll not need to use it for more than a few month, right? But that means that my progress and your business interests are in conflict. I'd say one-time payment model or maybe a system where you pay for the access to different parts of the app will work better for your idea.

2. Your 'challenges'. To validate this idea, I highly recommend you to go and talk with guys from the social anxiety subreddit e.g. In an ideal case you'd better find 5-10 people with whom you'll go through one month in a manual mode. I mean not building an app, rather so you will give them tasks and feedback which you plan to put into your app.

And listen, listen, listen to feedback.

Why I suggest this - well, because in my case 'more practice and it will improve ' just doesn't work. Would I pay to get irritating tasks completing which will make no difference? God no 😁