I've found and have now experimented with a program called Animation Paper, and I really, really like it. It has the onion skin functionality I crave, multiple colour layers per frame, fairly robust drawing tools (though no colouring tools just yet) and I will absolutely be using this software for evil. Keep an eye out.
I still think it's unlikely I'll take animation commissions because of the sheer work involved in making one, but we'll see. Maybe this software will get my time down to a reasonable ... uh, time, per animation if people don't mind sketch animation.
Anyway, here's Ookami running.

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Fancy keeping up with the foolishness that goes on at TRW Industries, including access to full-resolution files*? Now, you can! Information on how to join the TRW Industries Discord can be found here!) (*FA now resizes permanently. Some large pieces will be resized on FA.)