Pixelfed won't launch groups anytime soon unfortunately, I've followed closely and it seems it'll be out about the same time as Tesla's full self driving.
I think the best way would be choose yourself piefed or Lemmy, and then you offer mastodon or pixelfed as places for your users to join and subscribe. So it's a little more natural to them.
While your goal is noble, reality is that I would expect a very small percentage of people to migrate. Less than one maybe even. I think that even a few hundred more people joining the fediverse here is a great idea, but I don't want you to be discouraged when it doesn't turn out the way you hope. People love their platforms. (I also just read your last paragraph after writing this, my bad, glad you're in the right headspace :)