20k tracks is near the queue size limit for Sonos, but YTM is such a poorly implemented service I would imagin its limit is a lot lower.
Shuffle is available in two ways: one is to shuffle the queue itself, which is always available when the queue is active, and the other is via the “Play all / Shuffle” button added at the top of music service-provided lists. Those buttons will only be added by the Sonos app if the service provides the correct metadata. If they are missing, please be specific about which section of the YTM tree they are missing.
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