What to do in the first ten minutes
Leave GA4 exactly where it is. Add the Tiny Funnel script tag next to it, then open the recorder and click through your own signup the way a customer would: the landing page, the pricing page, the signup form, the welcome screen. That click-through is the funnel. There is no taxonomy to design first and nothing to name.
Then wait a day and compare the two. The top of the funnel is where they will disagree most, because that is where the cookie banner sits, and the bottom is where they should agree closely, because by then every visitor is a signed-in human doing something deliberate.
The one thing worth knowing before you compare
A GA4 funnel counts events. A Tiny Funnel funnel counts visitors reaching a step, and a step can be a URL, a page state, or an element on the page containing particular text. Those are different questions, so two tools can both be right and still print different numbers.
The case where it matters most is a purchase your backend confirms rather than your browser. In GA4 that means Measurement Protocol and a piece of engineering work to tie the hit back to a session. In Tiny Funnel you post the event with your own user id and it lands on that visitor's journey, next to everything they did on the way there.
The short version
Keep GA4 for the site. Use this for the funnel. They cost $10 a month together and they disagree in a way that tells you something.