There's Lulu based on Apple's Network Extension framework but there are quite a few issues with the framework itself (it will briefly initiate a connection even though there's a deny rule for that address etc).

https://github.com/objective-see/LuLu

I tried LuLu and it was okay but I did end up trying and subsequently buying Little Snitch. The level of UX polish wasn’t quite there for me with LuLu.

I also feel the same way re: UX polish. I haven't bought little snitch yet, but was kind of wondering if I even needed to. I've already got a pihole on my tailnet that blocks a fair amount of things, and then ublock origin on firefox to boot. If LS were only like $20 I'd probably just buy it for the pleasing graphs, but otherwise I'm not sure what extra value it adds. There's probably a usecase for it given the other things I have, but perhaps I'm not the target audience.

I felt similarly, waited a long time on the trial, and grabbed it on sale for a good bit off retail after I was sure that it met my needs.

Yes, there is: Lulu at https://objective-see.org/products/lulu.html.