If this is purely a proof of concept, I'd go with WordPress. So much of what you'd need in an app platform is already there, plugins for anything & everything are plentiful, and it's incredibly easy to add any necessary custom functionality in via PHP. Depending on what you are building, you might not even need to build from scratch if it works.

Plugins like Advanced Custom Fields Pro, Gravity Forms, Custom Post Type UI, WooCommerce, and MemberPress might come in handy.