What's the best way to build a MVP web app that handles order management, purchasing, invoicing, supplier management and inventory?

I've built a network of eCommerce sites using WordPress Multi-site. My customers manufacture simple products and I'd like to build them a more robust web app that pulls in data using the wooCommerce API or upcoming WP-API. I'm new to programming and taught myself enough to setup my server (LEMP), build a custom theme and the multi-site network... I'm wondering if theres an open source solution that I can hack a bit to get the basic MVP built. I'm not enjoying PHP and so open to suggestions.