I looked in to this a little when Windmill was last discussed here, and the gap you may bump into (as a replacement for Rundeck) is that AFAICT Windmill has no support for executing scripts remotely over (e.g.) SSH and passing the GUI values along as environment variables.
It seems you need a full-blown install of Windmill on each host where actions are to be run.
It's a good question, and deserves a more detailed answer than I'm able to give without more first-hand experience with trying to use Windmill in this way. But my sense of it is that there's likely to be a chunk of legwork involved in the passing of parameters and credentially over to the remote host, capturing both stdout and stderr from the remote site - things that Rundeck handles pretty much seamlessly - and a concern that using Windmill like this the question of which host the job is executing on isn't a first-class concept, like it is in rundeck.
It's likely that in time I will evaluate Windmill to get a more detailed understanding of this - there's definitely aspects of it that are very substantial improvements on what can be done in Rundeck.
But inevitably, the question of whether to learn & install & maintain two separate systems side by side, with some degree of overlap in their functionality, vs. just picking one system to do the whole job and living with the shortcomings of whatever it doesn't do so well - it's always going to be a difficult one to weigh up.
It seems you need a full-blown install of Windmill on each host where actions are to be run.