Hi Joeri,
Sorry I don’t really know anything about ADFS or how it is configured.
As you say the built in http challenge server should solve the problem without needing IIS, unless it’s prevented from doing so (i.e. something else that doesn’t use http.sys like apache or nginx is consuming port 80, or ADFS has it’s own http listener that is consuming all requests).
If it’s too difficult to get http validation working you can switch to DNS validation (under Authorization), ither using a supported DNS API or by scripting the API updates yourself (https://docs.certifytheweb.com/docs/dns-scripting.html)