I’m afraid all our servers run on Windows and use IIS ;)

Caddy runs great on Windows! But the integration with OS-specific features might not be super tight. What do you use from IIS that only IIS does?

Running 2 sites that use ASP.NET Webforms (in case you don’t know it, an 18-year-old technology that has a lot of problems and only runs on .NET Framework). I don’t think it’s a good fit for Caddy ;)

Caddy can sit in front of it as a proxy, no problems. It can terminate TLS for you, and your existing server can do the rest of the heavy lifting for serving your site.