While I haven’t personally tried it, I’ve heard people have issues with cooling when using the M.2 hat, especially when using their Pi for intensive applications (like hosting a Minecraft server). I’d honestly recommend just getting a 2.5" USB drive enclosure and an SSD. Costs about the same amount of money without the drawback of poor cooling. You can use it with any case, since it just connects via USB. I have been running my Pi this way for years (in fact I have never used an SD card in it).
Just a thought, but since someone else in the thread said you can stream to Chromecast via VLC, you can desktop capture natively in VLC and stream that to your Chromecast. I can’t remember if the native capture can do sound or not, but if not, you can instead use OBS virtual cam (you’ll need v4l2loopback for the virtual cam to show up), and open that as a capture device in VLC. You should be able to attach an audio source to that as well. While I haven’t personally tested it with audio, I have used OBS virtual cam with VLC before, and it worked flawlessly for me. If you can’t find a more elegant solution, then it’s worth a shot to try and see if it works