I think you should shoot for an hour or two of valuable content onto Gumroad, set a fair price for it, maybe even give away all or part of your first course. Build an audience to give you some fans to sell the second course to. See how it goes.
You have no visibility to your courses' students or their contact info.
You can send a couple of "marketing emails" per month for each course. Inside those emails you may only have links that go back to Udemy, so they are used primarily to market your other courses on Udemy. If you try to include a non-Udemy URL, the system detects it and won't let you send.
Plus, someone on Twitter just pointed out that every one of my Udemy courses is available on multiple pirate sites. So, at this point, I think I'm just going to shut those courses down. I certainly won't be expanding them anymore.