+1 to getting on a call and recording it. Pop the recording into Otter for a transcript and edit away.
This has worked really well for me because of a few reasons:
1. What I've realised is that while these people are SMEs in their fields, they might not always be able to translate that expertise to a written format.
2. The unfortunate truth is that they look at filling a questionnaire / docs as a chore. But when they get into a conversation with you, they are more willing to share their knowledge.
3. Close ended questions restrict the SMEs to just the questions mentioned. When you get on a call, you get the opportunity to dig deeper into certain topics and have follow on questions if needed.