Also, it feels, bizarrely, like cheating.

But that's a good thing, isn't it? I mean, that's what rational Harry Potter would do.

Try finding a tutor that will let you make short videos of them doing the signs, and then you can put those videos into Anki. (At least I hope it is possible to put videos in Anki, never tried. Or maybe photos.) Could be more efficient than usual learning.

how to negotiate schedules and rates

Decide in advance how much is acceptable for you. Find out the minimum hourly wage in your area. Those are your limits. You can start by offering something in the middle of this interval.