If you want to only show the footer after the Flarum Single Page App finished loading, you will have to use the extension API and hook into the Flarum mount method or similar.
You could also delay it by a specific amount of time, but you won't be able to predict how long the Flarum SPA takes to render.
If your concern is that the footer appears too high on the page before being pushed down, maybe you can solve the issue by assigning a min-height
to the app container?