Just following up on my own note as I think I've fixed the SD thing. There are no directions or warnings anywhere, but now in my experience exFAT vs FAT32 formatting makes a huge difference. After wiping and formatting the card again today to FAT32, I saw my copy task take about half the time (roughly 3 hours for over 300GB) that it took previously and while there was one stall on the library update it passed pretty quickly and then the device seemed to complete the mission. whew!
By default Windows 10 will format a 400GB card to exFAT. Curious whether I'll see any files when hooked to USB in the car now as well though the car I mostly drive will be connected over Bluetooth as it long pre-dates CarPlay / Android Auto. I have the
Earstudio ES100 coming for LDAC connectivity.