Gumroad is going to handle payments for you.
You mentioned a course. If you plan to offer video streaming, you'd have to come up with something to provide that platform.
There's places like Teachable and Thinkific for that. That's what I did initially and now years later I want to build my own platform because there's a massive laundry list of things that I don't like about those existing platforms.
But now it's difficult to move an audience from platform A to B because they expect a working and fairly feature rich solution since that's what they are used to. It means the custom solution needs to be shipped as something more than an MVP, which makes it take longer to develop. At the same time, like you said, building the platform takes away from creating the content.
If I could go back in the time I would have built my own platform first while testing the demand with free videos on Youtube.