I recommend developing a short survey with an incentive for completion, optional contact info questions at the end + interest in participating in an online focus group in the future. Then requesting the opportunity to post on private group boards dedicated to either LearnDash users *OR* PK-12 teachers on Facebook, etcI found this private group you could approach on Facebook: "LearnDash LMS Tips & Tricks" (14.5K Members). The Census will tell you the largest # of school districts (i.e. large # of teachers) https://www.census.gov/library/visualizations/2019/comm/largest-school-districts.html. Google search to see if LearnDash is included as a tool for teachers on the school district's website. Then prioritize finding groups of teachers online for those school districts first in your survey outreach.
Note: many groups on FB are against promotion, so I strongly recommend framing the survey as research AND include an incentive for completion as an indication you value the respondents' time. If you'd like to schedule a short call with me to discuss this process further, please let me know. Best of luck!