No need for a separate license. All manufacturers of "tobacco products" (which includes anyone who mixes juice for commercial purposes) were supposed to have registered with the FDA shortly after the Deeming went into effect. I don't know the exact date, and I think it may have gotten a few, relatively short extensions, but I'm confident that part of the Deeming is in effect now. So with a few key-strokes, the FDA could issue "guidance" that nic base (dangerous stuff that it is!) may henceforth only be sold to registered manufacturers.It always seemed like nic might be treated differently to everything else. They can jump on hardware and juice like they are talking about in the House and Senate. Nicotine has to be available for cigarette manufacture. They would need a separate remedy, like making it available to licensed buyers only. It could happen tomorrow, outside of the other legislation that's going on.
However: My guess is they won't do this in the near future, because nobody is currently up in arms about us DIY mixers. Now if/when they really clamp down on the industry (2021?) and an informal market for juice emerges, then I could see it.
That said, I still think it would be foolish to wait until the poop hits the fan. If you're not sure you've got enough, you probably don't.