Which 3rd party for Backend Art Estimation App

Hello everyone,
I don’t have a lot of technical skills but I’m hungry and I learn fast.
I already did 3 small portfolio type websites with a small embed code for external video integration.

I’m at the first stage of designing an art estimation website for a member of my family,
My plan is to use webflow for web and Drafbit for mobile.
I love webflow for the easy designing and draftbit looks pretty solid.

I’m currently wondering about the backend solution I need to use.
The web and mobile app needs the same fonctionalities and to use the same backend.
The app needs these functionalities :
Customer account with login, possibility for the client to send pictures + text to estimate his art goods.
Everything should be then sent to an easy and good looking backend where the company can estimate the good and send an estimate back to the client maybe through a chat so we can offer to sell the good for the client and answer first questions before going through emails.

Thank you in advance for any tracks you can give me !