I have the same problem and my colleages share these worries as well. Any tips for open-source alternatives? We tried a couple of WebRTC things with screen sharing but the image quality is bad (which is expected when using a lossy codec meant for photos/movies to compress screen with tiny details like fonts etc.). Now we're using VNC tunneled through SSH but the setup is a little difficult.
I'd love to try something better but running a closed-source app with complete access to my screen, mouse and keyboard connected to the Internet is really not an option. And it's really difficult for me to understand how so many people have no problem trusting it. What am I missing?
I don't think you're missing anything, except maybe just that closed-source is the default on macOS and Windows so the marginal loss of safety from one more closed-source privileged app isn't so high there.
For the moment maybe the best we can do is trying to use apps with screen sharing support included, e.g. Visual Studio Code's sharing.
