We are working on a system which retrieves data from customers' Shopify shops and provides some services based on this data. In order to make it as convenient as possible for an end-user we would like to update this data on a daily\weekly\monthly basis. For now we only came up with a solution of implementing unlisted app, prompt a user to provide all necessary permissions for the app to access their shops and fetch the data. But the token we get doesn't seem to be valid for a long time and we probably won't be able to reuse it a day later. We appreciate it if you can share any success cases of implementing this kind of approach.
You provide an App to the merchant they can install using oAuth. When the merchant is prompted to approve the App, Shopify will then provide your App with a long-lived access token you can use as much as you want, for as long as you want. I use a custom App from my Partner App dashboard to create these kinds of one-off Apps. It is superior to the one where the merchant has to tick off scopes and permissions IMO.
There are two kinds of token you can ask for and receive. One is considered for offline access, or long-lived. It works for everything. It is for webhooks as an example, or other access where no person is involved. But, there is also, online access tokens! Say a person clicks into the App from Shopify to do some work. You can request an online token for them to do their thing, and that token is only good for say 24 hours.
So you have options!