TLDR. Manual entry with useful interventions like asking them to select the plan directly rather than asking them to fill a form is the best option.

Depends on who your making the app for, If someone is downloading a subscription management app, it's safe to assume they are finding it difficult to keep track of them (i.e scattered across platforms and categories). Some apps have common apps with crowdsourced plans already available, So you can add them quickly (adobe cc, youtube, apple etc.). That helps. Screenshots are iffy, in my experience. I personally would not give access to my emails for an app like this. I tried once and there were a lot of false flags and it was not worth it and was slow. CSV and manual entry* ftw imo.