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.
I recommend Node.js because you are creating a new application and will likely not have a huge workload, at least not initially. Node is incredibly fast and used in more SPA's than any other solution, but as the CPU load increases, the efficiency starts to drop off.
I could see how scalability could eventually become a concern if your SaaS platform becomes extremely popular, but considering that you already know Node, I think it would be wiser to shorten your time to market and develop a quality product rather comfortably.
Otherwise you also run the risk of security flaws due to inexperience with a new framework and that is not something you want when you are handling other people's data.
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.