Side thought: If this takes off as a popular quality implementation, an additional effect might might be that it's easier for vendors of other services to integrate with users of your software. Maybe there's some way you can profit from that savings or reduced sales friction. (I've had to implement several F500 SSO integrations from scratch, because they were doing bespoke/custom things, and even "SAML" doesn't necessarily interoperate, but software like yours might out of the box.)
Question: For the free hosted SSO, how well are you going to be able to secure that, so that your customers aren't compromised through you?
Question: Will the free tier SSO have uptime guarantees, since it'll be a single point of failure for all your customers? For startups that decide they'd like it hosted for them, but need an SLA, do you expect to be able to provide that at a price doable by startups? (Will a cloud provider pick up those customers using your software?)