hi,
I'm web developer and have strong experience in web development because it's my route work.
how i will solve your problem?
after submission form, user not direct to thank you page because redirection are not properly work due to invalid condition.
Ao i will check your error and apply a valid condition, when user successfully submit form, if all the fields that your entered are correct and data save into database than redirect to thank you page.
i also able to set your another isseu, while payment successfully done (using strip) then send a confirmation email address to user.
I know im new in here but i also Attempt these types of tasks many time, so i easily solve your problem with 1-2 days..
for any question, inboc