Homestead/private estate venues

I would try airbnb. There's an option where you can filter properties that are suitable (and allow) events.

It will be pretty hard to find an airbnb or vrbo that allows events, at least in the more heavily populated counties. Shupe Homestead in Hygiene (outside of Boulder/Longmont) allowed you to bring in your own food & drinks and do a lot of DIY. It had nice views too. I'm not sure about the end of the event though. Lionscrest Manor was a private estate so they let you end later but I think they had preferred caterers. I read an article out of Larimer County that was a bride & groom who were 30 days out from their wedding at a local farm and the county denied the event permit because the neighbors complained about weddings. The "venue" refunded their money but with 30 days to go I'm not sure what you'd do.

This is exactly what we wanted as well and had no luck. We tried AirBNB, VRBO, Bed & Breakfast, Inns....you name it, we tried it. Depending on your guest count, you may have more luck than we did but with 50-75 expected out of 90, the main issue we ran into was having enough space for parking. Good luck!!