Sounds good — take your time and test with the full file, that’s usually where the fake “it works” results fall apart. Short clips can hide buffering, format quirks, or fallback paths way too easily.
When you’re back after coffee, please note the exact HEVC profile/bit depth/chroma if you can, plus whether Chrome/Firefox/mpv behave the same. If HEVC is really clean now and VP9.2 is stable, that’s a big step forward for the RK3588 browser story.