I have the same problem, whether using a Chromecast or a Samsung Galaxy S10 Ultra as a client.

I got as far as "It looks like it is using software encoding and it should be doing nvenc or something, but it Works Fine for everyone else so nobody ever writes about how any of this shit actually fucking works so troubleshooting it is more effort than it is worth."

And it isn't clear to me -- at all -- how plugging in a dongle that acts like a monitor is going to help anything at all.

I already have three fucking monitors plugged in. All different shapes. A fourth one cannot help.