I don't know what correspondence courses are like today, since the invention of the internet, but it might be worthwhile to look into it. Back in the day I took two TV repair courses that actually gave me a pretty good understanding of electronics and electron theory, which helped a lot in my later career. Nothing was done online. All the books were sent through the mail along with a lot of equipment for hands-on learning. I received the parts to build a VTVM (vacuum tube volt meter(?)), an oscilloscope, a vectorscope, 2 TVs, and some other things. I have no idea what a course like that would cost today, or if they even exist, but it was definitely worthwhile for me.