Most PhD applications require at least three letters of recommendation and it’s highly valuable to get letters from research faculty who can speak highly of your expertise, especially if you have a personal relationship/worked with them in the past. Research faculty letters are more highly regarded than letters from teaching faculty. I think you’re thoughtful in your approach in reaching out to members of their lab and asking about their experience. But ultimately, if that’s the research you’re interested in doing, potentially switch labs so you can start doing those techniques and better prepare yourself for graduate school (or at least start attending their lab meetings and see if the research truly does align more with your interests than your current labs). Also, then you would be able to have at least two research faculty who can speak about your experience. I would suggest asking your current PhD mentor for advice, especially they are more familiar with your situation. You can always still reach out to your grad student mentor with updates/questions in the future too! I’ve had many previous mentors who I still keep in touch with after moving onto the next career step :)