I think many problems are like this. We engineers to think that we have to build something "rocket science", but actually many businesses don't require "rocket science" software to take off. It's more about the execution.
>I'm relatively new at modern advanced web apps, so things like taking payments, handling customers, integrating to a customer's on-premise or cloud based authentication
These days, there are almost already SaaS for everything. A little googling here and there will find you what you need :) Also sites like indiehackers.com is really awesome. People share what indie projects they had built, there's also a comment section where people can ask.
>Or would I be better off finding a business partner that has been through all this already?
I think you should try to do this in parallel with trying to learn it yourself. Finding a good, competent, and suitable business partner isn't exactly the easiest thing to do.
I would even go further than that, that's exactly the reason why Wordpress and basic CMS are very popular, most companies have some very basic and similar IT needs.