That is a perfectly good reason! It’s really nice to see when handlers make decisions based on supporting longevity in the activity 🙂
River did improve throughout the sessions in both videos so you are definitely on the right track and she is understanding. Keep shaping it in little increments. A clicker would be very handy here if she is used to it. We know you use a verbal marker most of the time, but if she is clicker savvy, using it for just a session or two might accelerate her understanding.
Are you shaping for a single paw touch, repeated paw touch or a paw touch with duration? Knowing this will give direction to your shaping sessions.
Another suggestion that you and River can play with is teaching her to give the paw behavior you eventually want using a container lid (like a yogurt container). This exercise doesn’t’ involve truffles. Just you, the lid (paw target) and some treats. You could even use a post it note if you like. I prefer the lid because it can get wet (making the tool more mobile for outdoor activities). Set the lid on the ground and mark & reward for showing interest, moving toward it and eventually stepping on it with one paw. Once she can reliably do that, start to move it around, rewarding only for the paw behavior you ultimately want. If you want a sustained paw target, you can build duration this way too. If you just want a light touch, time your markers so it is the moment just before she makes contact. Once you have the behavior on this lid, you will transfer it to a truffle search by pulling out the lid for the “show me” part of the scenario, bridging the gap between the “trick” you taught her with the lid and her alert on the truffle.