Iflier, welcome. As said, you've set the bar really high for yourself. I would never want to say that something can't be done, maybe it can. But the competition for a TP position is big, and there are many very well qualified and experienced pilots from whom to draw.
I have worked and flown with quite a few test pilots. Each one I can think of was chosen, rather than choosing that role. The test flying I have done similarly found me, rather than me applying for it. I was proposed as being the most suitable person based upon my experience. For my experience, there is not a tremendous amount of work out there for many test pilots, and I know a few who have retired, or gone back to commercial, to keep earning. For myself, I would only fly 20 to 40 hours a year of flight test. The rest of my flying is recreational, or personal transportation. You can't make a living, much less return the investment in training, on 40 hours a year of working that job.... My success has come from flying types which are little known to the ex military TP's (like floats and taildraggers).
So you have to have the piloting experience first. For what I have seen, several thousand hours on the class of aircraft for civil test flying. And you'll need a keen awareness of the test methods and certification criteria. Start reading the books on the sticky thread in this forum.
It can be done, but you'll have to put so much into it, you may not be able to see the end point for a while, as you work toward it.... But everything starts somewhere...
As a mentor used to say: "It's amazing what you can do, when you don't know what you can't do!".