You could just sign everyone up and pay for it out of taxes one way or another. I'm flexible about how exactly it's implemented after that, but the biggest absurdity in all of these plans it that you have to add additional complexity to the tax code, and a ridiculous additional layer of adminstration/bureaucracy. If you want everyone to sign up, don't "mandate" that they "buy in" to the program. Just, you know, sign them up and take it from their paycheck. If they don't have a paycheck, they're still signed up.
You shouldn't have to make people, for yet another additional program, have to jump through hoops to prove that they're poor enough to be eligible for a subsidy, or to fill out yet another set of forms to get yet another refundable tax credit.
Just sign them up. Pay for it. There's no reason not to do it that way. Medicare for all, baby.
...adding, while I'm not thrilled about the idea of keeping our insurance companies around I don't think they have to be driven into the sea. Some countries have basic universal plans which include highly regulated private insurance companies as part of the mix. True universality in that people are essentially automatically enrolled (implicit in that is portability of course) and community rating such that the factors which can influence price are severely limited if not completely done away with are necessary. You could default people into Medicare and let insurance companies compete with that to lure customers away, or whatever. The details of course matter, but this absurd extra step of "mandating" that people "buy in" is just dumb. There's no reason for it.