I should say that the Music app seems to have calmed down re its use of memory now. I wonder if, like those people who've managed the updating process, the sequence of crashes I experienced were the app sorting itself out.
So... still haven't moved the artwork file or indeed any others and things seem to be ok.
Memory use with Music idle is currently 288.MB, playing a track it rises to 365MB. Interestingly, randomly and manually playing tracks from random albums in Album view has cranked the memory usage up to 1.32GB after about 10 or 11 selections. I left Music on shuffle for most of the evening 2 nights ago and woke up to find the computer had crashed.
I feel like I'm having to coax Music along to make sure its running anywhere near properly. Can't be a good selling point for Apple at this stage! I wouldn't buy Apple Music under any circumstances on this basis - Tidal, Spottie and Amazon Unlimited look far more stable. Am looking into other digital players/databases my only concern is that, like Plex, they'll only work with one outboard set of speakers at a time over Airplay as opposed to Music/iTunes capacity to use multiple outboard speakers