I’ll reach out shortly. My thoughts on this are you don’t remain free, but instead charge based on a cost recovery model. You figure out annual people/tech/admin expenses, forecast and observe request volume over time, and then adjust per signing request pricing accordingly (or perhaps sell buckets of requests to high volume consumers, contracts ensure smooth cashflow). This enables longevity and stability of the service (which gives warm fuzzies to consumers of it), no concern of an acquisition or buyout, while enabling servers to spin and people to eat.

TLDR think electric cooperative or similar. You’re building an internet utility/primitive for long term consumption.