How do I identify the first page a user lands on, who eventually completes a goal, but in a different session?
You’d have to drop a cookie on the first session and read it on the concerting session. GDPR will make it more difficult again as you need permission to drop the cookie.
Unfortunately, that’s not really possible with default GA as there are a lot of factors such as different browsers, devices, incognito, cleared cache, etc. The best GA provides is Attribution modeling that allows a comparison between First Interaction and Last Interaction, which will receive the credit. It’s more about giving you an “idea” of how your various channels are supporting your overall performance. User ID definitely assists with this but only with Google users that are logged into their account.