Route two
Forward each submission from a webhook
Add a custom webhook to the same form and Upform POSTs the full response as JSON to your HTTPS endpoint on every submit. Your endpoint can then send its own event to GA4 through Google's Measurement Protocol, which counts every submission, including people who close the tab on the Thank You screen.
The catch: you write and host that code, and the payload carries the form, the submission and the answers, not a Google Analytics client ID, so the event will not be tied to the visitor's earlier session.
Set up a form webhook