Honestly I don't think selecting the quality on my phone makes a
difference; it seems to just be stopping/starting casting that makes the
chromecast "realize" that it can stream in higher quality.
Having very similar issues on a Chromecast ultra, connected via Ethernet
(through the USB plug), on a 1200 down/40 up (Mbps) internet connection.
About 40% of the time, it defaults to playing content at 480p, which
looks like hot garbage on a 4k 65 i...