I'd say something along the lines of:
We're currently as a crucial inflection point in the journey of our company, and we'd love to serve you better and provide solutions faster and better to you in the coming years. As part of this journey, we're currently in talks with taking on venture capital funding that would give us the funding necessary for this next chapter. And the investors we've been talking to were hoping to better understand the product and its use cases from a customer perspective, like yourself. As you are one of our founding customers, an honor we do not take lightly, and we thank you for all the support you've given us so far, we couldn't think of a better person to have that conversation with our prospective investors.
We understand you're really busy, and this will likely be a 20-30 minute call. But if you're open to it, we'd love to put you in touch.
Credit: David Zhou