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).
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.