Have you looked into state parks or nature preserves in your area? They're often way cheaper than you'd expect and usually have built-in backdrops that don't need much decorating.
I'd actually push back a bit on the barn idea though - while they look great, they can get pricey once you factor in rentals, insurance, and the fact that they're suddenly trendy. For your budget with 80 people, you might find more breathing room with a community center, church fellowship hall, or even a scenic public garden that allows events.
The key is picking a space that already has character so you're not spending money trying to make it Instagram-worthy.
With 15k split across venue, catering, and everything else, you really need walls and a roof already doing visual work for you rather than starting from a blank concrete room.