I think a few more details are needed to get you a clear alternative:

• Which devices you want to cast from? what content you want to cast (DRM content like netflix, and/or your own media)? what kind of TV you have?

In my case after degoogling, I use mainly apple devices besides my windows pc. 2 of my TVs have AirPlay built in, so there’s no issue casting anything. If your TV is rather recent it’s likely to have it too.

The third TV is tricky, it’s an older 4k LG. I have a linux box connected to it and installed UxPlay in it. It only works with AirPlay “mirroring” so you kinda need an app that can treat the TV as a second monitor. Otherwise the mirroring won’t cover the entire TV screen. I’m still assuming apple devices here, but there’s OutPlayer and nPlayer in the appstore that can do this. It does support sound-only casting, so if it’s music you want it should be able to direct cast from your apps.

The second caveat for UxPlay is that it only works with DRM-free content (youtube, self hosted media). For DRM content I haven’t found a nice alternative for the old TV , so I use its built in apps (netflix, amazon prime). Kodi exists, but the plug-ins support for streaming sites isn’t good, often getting stuck to low-res content.

I’m guessing buying an apple tv/fire stick/roku is the only alternative for DRM content casting. I also explored the idea of “degoogling” my unused chromecast 3rd gen, but absolutely nothing exists for this and it just collects dust in a drawer.