Should ask yourself what it is that primarily attracts you to this field. working with hands making stuff, in which case I would get a real job and set up a home shop. Or if you really want to earn a living with bikes. If the latter your guess is as good as mine, but I would probably put a lot of effort into meeting people who buy bikes and learning how to network and sell. I have a friend who normally sells more stuff in the first 3 days he starts in a craft, than I do in a lifetime. He just talks to everyone about what he is doing and ropes people in. While I would generally prefer my privacy.

So you need to figure out who buys high end bikes in your comunity, and how you can get in front of those people. Networking, advertising, comunity work, clubs, schools, libraries, teach a course, etc... Making frames never made anyone any money, only selling them, or better still selling the lifestyle on a T-shirt. It isn't just the money that you get by going in search of clients, but the interesting work. I'm not suggesting doing this before you know what you are doing, but I would suggest keeping your eye on who it is that will buy this stuff and what they want.

Take out a sheet of paper and write down the names of all the people you know who would like to buy a custom frame, and then figure out how many such frames you need to sell in a year. You won't just be selling frames, and you won't be able to make a living at first, but if you can't come up with any names, that could be a problem. Also worth considering where these people live geographically. I'm guessing possibly not Detroit. You need a place where they exist, where the economy is strong, and where competition is reasonable. A lot of other people doing the same thing can be good, but a lot of people doing it part time, and selling at unreal prices is bad.

In learning to make frames I would focus on design, joinery and tube fitting. I learn design, for at least the stuff I am interested in by just doing cad designs of existing bikes, and the bikes I wanted to build. Trying to draw stuff in real space will allow you to build up a file of components, and learn what you don't know and need to learn about how and why bikes are pt together as they are. Do lots of practice joints, for welding as many as 100. I you have never welder or brazed, then maybe you should start out on flat stock of a similar size. Tubefitting is the easy part, at least for me. You can make things easier for yourself through designs, but I never had any problem with this, and didn't have to practice it.