I am starting a new project to build a simple ERP system for small businesses, where the owners can also manage orders on their phones.
So I'm a little confused. Please advice.
Will you build it from scratch? There are some open source ERP/CRM solutions that you can use as a base for your solution. SugarCrm is an example. By looking at those, you can then decide which language you'll use for the backend.
There is no problem to keep using node.js for your backend. Keep in mind that you already have expertise in it, so you could focus on development instead of to learn a new syntax/framework. There are good libraries in node.js that could help you in the development (services, validations, integrations, etc) also keeps you with a single language to the whole system. Django, as far as I know, it will provide a solid base for you, but it could be too much for your purpose, also could be more complex than you could need. Java provides to you many frameworks to simplify your integrations also could achieve a good performance. Anyway, I recommend you to follow using node.js, since you already know the syntax/platform.