Garmin and Polar take data seriously. I use a polar and don’t mind using their app. They have a setting to delete data after a certain time limit. I have mine set to 2 years.

No clue. I don’t use any consoles or steam.

You could buy a cheap(ish) device dedicated for this purpose that can handle the apps you want. Think of it as a more expensive chromecast.

A separate device can also mean more privacy depending on how you configure it.

My iPhone has a screen mirroring function. If the TV does Bluetooth, you can display your entire screen and play whatever you want.

I think Android must have a similar feature, I’ve seen connection popups for Samsung devices that I denied from my TV.

If you have a non Bluetooth TV, then it won’t work.