This is not what you are going to want to hear but this is my personal experience.
I can improvise and play solos on the piano and I can sort of fumble around and solo on guitar. I’ve been playing the banjo for three years now and only recently have I been able to noodle around a bit, which for me is the first baby steps to being able to improvise and solo.
I don’t think there is a ‘method’ or ‘primer’ for soloing on the banjo (though maybe I’m wrong and someone will come up with something brilliant here).
In my experience, soloing as all about being able to play what you hear in your head. So you just play the notes you hear in your head as the song is going along. It’s pretty easy to say but being able to do this involves quite a lot. You need to be able to play your instrument. Then the licks and phrases and scales just become the tools you use to do this.
For instance, learning scales teaches you where to put your fingers when you want to play a certain note (or a string of notes). A lot of solos are just bits of scales stuck together. But when you solo, you’re not really taking bits of scales and sticking them together any old way, you’re hearing this collection of notes in your head and then playing them.
When I was learning the piano, I asked my teacher to teach me how to solo. He gave me this book with blues piano solos. They were written down and so I learned them. After that, I was able to play a ‘solo’ but every time I played it, it was the same ‘solo’. It takes more than that to be able to really improvise and solo.
You need to be able to make your hands do the things you want them to. Then you will be able to play the notes you hear in your head using scales, fretboard geography, licks and phrases.
When I was learning how to improvise on the piano, I would spend hours just playing whatever my hands wanted to play (I could play fairly well at that point). Most of it sounded pretty terrible but I would start to hear things I liked, I would start to play things I heard.
So now, on the banjo, I’m starting to do this, noodling around. For me, it’s the first steps.
Sorry that this has been so very long. I hope there are at least some useful insights in there.