Need Advice on Budget-Friendly Ceiling Speaker Setup for Pre-Wired Airbnb

Hey everyone! I could use a little guidance from the home audio crowd here. I’ve got an cabin in the Smokies that came pre-wired with ceiling speaker holes. I'm going to be putting the place on airbnb. I’m trying to do this as cost-effectively as possible because the cabin is small, and I don’t need a super high-end system. Honestly, I wouldn't even put speakers in if there weren't holes cut into my ceiling already.

I had the guy who installed the previous system come out, and he's quoting me really high prices ($15K-$20K) and trying to sell me gear from dealer-only sites like SnapAV. I feel like he's upselling me on stuff I might not need like running ethernet cables through a router and switch to every TV when I have wifi that has 250mb download speed at the furthest location.

My main questions for the community:

1. The ceiling has speaker rings for episode speakers. Does anyone know if Sonance speakers from Best Buy will fit those rings? It seems like a simple question, but the installer won’t give me a straight answer.
2. Is it realistic to do this as a DIY project? Everything is pre-wired, and it feels like I could just install the speakers and amps myself. I've gotten a quote from the guy that installed the system for the previous owner so I know exactly what was in the system.
3. Any tips on where to get this kind of gear without the dealer markup? Everything in the previous system is from snapAV. I’d love to know if I can just buy the equipment myself and not deal with hidden pricing.

Thanks in advance for any advice!