I'd lean toward letting the player's results decide when the guide fades, rather than a fixed count. Something like: full outline at first, then after two or three clean casts it drops to a faint ghost, and if they miss a couple in a row it comes back on its own. That way nobody gets stuck in a memory test, and the ones who've got it aren't staring at a prompt mid-fight.
Your "do something else, then use it again" idea is the right check in my opinion. Recall after a short break tells you way more than five traces in a row, which mostly measures copying.
For telling learning from copying, I'd log how close the stroke is when the guide is faint or hidden versus when it's fully visible. If accuracy barely drops without the outline, they've learned it. A toggle in settings to keep the guide on is also nice for players who just want it there.
Curious how forgiving your rune recognition is, since that changes how strict the fade should be.