I’m not aware of any standarized best practices, but I’d certainly like to see them :).
As you mentioned, one issue is that users may not be familiar with Solid that much, so I’m hesitant in mentioning “issue provider” or even “webId” in the UI. That’s why I decided to just say “Log in with Solid” and I try to make that work with anything they type. Ideally, I’d just ask for the issue provider, because that’s what the authentication library needs. However, I think for users it’s easier to write their storage url or webId. So what I try to do is, with the url they type, search for a webId and if I find it, read the issue provider from there. If I don’t, I just pass whatever url they typed directly into the authentication library.
I’ve always thought the best UX would be to have a browser extension that users have to manage their Solid identities, and with that they won’t have to type anything. For example, many apps in the web3 space use MetaMask to log in with a single click.