I created the open source tunnel project called inlets which now has 8k GitHub stars, the same thing happened - lots of people used it, got value and didn't pay a sponsorship or for support.
So seeing the problem, I then went on to create a paid version which adds value and features for companies. The main issue was that many developers anchored the pricing to ngrok, but ngrok is not the same product, for one, it's a SaaS with rate-limits and is a cloud service.
inlets PRO doesn't have any rate-limiting, can act as a VPN, integrates natively with Docekr + Kubernetes and can be self-hosted too. That's the value of it. So when asked, I advise Ngrok customers to stay with what they know, they aren't the target market.
It still makes anchoring a challenge though, and Neil Davidson of Redgate talks about this in Don't Roll the Dice - another eBook the author may enjoy reading - https://www.red-gate.com/library/dont-just-roll-the-dice