If I'm going to put in the effort of tracking down the expert who does/did "X thing", I want to make it count. Every question has a backstory, and providing that context eats precious time.
What are some ways that you (in the role of the expert) have found efficiently get to the point where the founders are at in their thinking?
There is no exact formula but just some quick thoughts you might find useful.
I would consider sharing a list of a questions with the expert ahead of the meeting (like an agenda) so they know and can think ahead of time of what their responses might be. Also, point out things like wanting to be "a good steward of your time" so they realize you are being conscious of their time commitment to the meeting. Lastly, always leave yourself time at the end for scheduling a next step/follow-up with a specific ask (e.g. could we touch base again via a cal in 2 weeks etc.)
Start with where you hope this expert's advice will get you. Be as specific as possible about as many details as possible. Then walk backward during your conversation to where you are now. I'm sure the expert knows a lot of things, about a lot of things. Your challenge in structuring the conversation is to laser focus on those things that will have a practical impact on your organization.
Avoid yes/no questions. Ask questions about How and Why.
One way to do this is to prepare your questions before hand, then review them considering why you are asking each question: this may help you get to more salient questions underneath.