WP_Redirect using shortcode for form

Hi I’m having problems with wp_redirect in a custom plugin I’ve written. I know it’s because content is being served before the redirect, but not sure how to correct this because it’s in a shortcode. Could anyone point me in the right direction?
Essentially once the form’s been successfully filled out, and the data inserted, I want to redirect to a thankyou page.
I’ve copied the shortcode function here:

https://pastebin.com/gKL7FX78