There's no "online university course" that's going to help you that much. Getting a proper bachelor's degree means attending an accredited university and majoring in a subject. Aside from your major courses, you have to take a variety of courses that involve sciences, humanities, social studies, English, and so on. It's called general education requirements (GER). There are bachelor's degrees you can supposedly earn on the internet, but I've never seen any that involve learning Japanese or teaching English to Japanese students.
Any school you attend must be recognized and accredited by one of the regional accreditation organizations on this page (if you're in the US) ->
Regional accreditation - Wikipedia, the free encyclopedia
A bachelor's degree that comes from somewhere else is not necessarily considered 100% legit.
My advice is to attend a real university that offers Japanese classes. You can major in TESL and minor in Japanese or something like that. It depends where you live and what universities are in your area. I'm not saying there's anything wrong with online courses, but they aren't going to be that practical if you're intending on moving to Japan. Besides, attending a university for 4 years will be a lot more fun and engaging than sitting at a computer screen for four years.