Temporal is MIT licensed and comes with multi-tenant security features and its durable execution model is solid and scalability is phenomenal. They upsell to the cloud offering and the default OSS auth plugin is intentionally limited (you might want to develop your own if you self-host). You'd probably only look at the Temporal UI when debugging.

Windmill is very cool, but it is only suitable for trusted teams due to its security model. If you want to be able to develop scripts and workflows in the web browser and run them together with trusted colleagues, on a schedule etc., then windmill might just be for you!