Ok I follow you now. I was confused by your 1st post. I used to cast everything from a laptop or PC prior to building a Plex server, which just runs on my main PC. I cast directly to Chromecast though without much issue.I'm only casting to one other device at a time during the tests not both and the secondary monitor from the PC casting was disabled running off a 3080.
Its a known issue with the Fire sticks and windows PC specifically. The Roku is not affected, the roku started casting fine after a factory reset on the vizio 4k tv but the difference in quality between dolby video and just hdr (not hdr+) is enough to toss it back to amazon. That and the off chance it starts crashing again when just trying to play 4k youtube videos.
updated the last post for clarity as well
I mirror my phone to the tv via Chromecast no problem. Cast my phone. Whatever I do on the phone from there goes to the tv. Sound, everything.I do cast through the browser sometimes however for full screen mirroring or other apps not supporting it would be miracast. TV has smartcast + airplay built into it. I ordered the roku streaming 4k 3820R which will have dolby vision and support miracast anytime without having to enable the service remotely. Hopefully no major issues come up as no other devices exist to fit the bill.
Some other roku facts if anyone else decides to purchase,
The 3821R (4K +) is a waste of money, only difference is same remote but with a rechargeable battery and I think a remote audio jack for nearly 2x the cost
The 3820R is the newest build that has dolby vision where as most older models before 2021 do not and are priced nearly the same. Only bestbuy has backorders for the model currently aside from amazon.com/ebay.