It's definitely a valid concern. I connected a device between it and the rest of my network and spent some time recording anything it sent. I wasn't able to find any telemetry at all. I also keep it segregated from the rest of my network by vlan, and it only has a few specific routes to reach my self hosted services.

I do, however, want to point out that a chromecast is going to have a ton of telemetry. It's hard to be truly certain of any devices these days, but I spent some time digging into the OS and I'm pretty confident that it's truly my device.

I don't use google cast, but I'm fairly sure it could if I used it. It has a remote, so I just use that. If I want to use jellyfin, or whatever app, I just install it on the device and log straight in. I don't use mine as a portable stick like you mentioned. It's intended to turn a dumb tv into a smart tv, and that's all I want it to do.