Actually, I spoke too soon! Configuration describes fails on reboot - partition still is automounted to /tmp/run/mountd/sda2 and NOT to my preferred /mnt/share!
I can manually get it where I want it by unmounting the wrong mount point then restarting the nas service under Luci System -> Startup screen. But I want it automatically.
My first “fix” attempt was simply to disable the mountd service (System --> Startup). Rebooting resulted in the partition where I wanted it - but now hot plugging a USB device into my other USB port won’t automount. Not optimal.
Then I found:
https://wiki.openwrt.org/doc/howto/writable_ntfs
which suggested an addition to the /etc/rc.local file to run an appropriate ntfs-3g command. I added it (modified to reflect my locations - /dev/sda2 and /mnt/share), rebooted - and it worked! Next, I re-enabled mountd, rebooted and tried again - Yay! It’s mounted where I want it and mountd is running so, presumably, it will hot plug a 2nd usb device appropriately (I haven’t tried this yet!)
So for now, it appears things are looking good - time to go on to getting samba access to this to move media files from my computer, then work on getting minidlna to work.