I don’t think you understood how the move data disk feature actually works. With the hybrid mode, your points 1 + 3 + 4 are irrelevant. And for 2, boot times are valid for HA OS only as HA Core is stored on the data disk, not the boot disk.
So please just read and understand what this topic is about. It is definitely not about a migration to a full SSD/NVMe setup.
And fingers crossed for your next OS upgrades - as mentioned, you might want to have a look at the GitHub issues for HA OS (but using an internal NVMe without a third party external case you might be good).
I‘m really curious to hear how things turned out for @rwelsh09 .
TL;DR:
I found an xhci_hcd error linked to an incompatible SSD enclosure, I switched to a better SATA to USB and retried. Initially encountering a supervisor error, a reboot resolved it, and the system was successfully up and running 14 minutes after starting the process.
Hello @e-raser & @FloatingBoater
Thanks for your replies. My original data disk was 27GB of my 32GB SD card.
After leaving the system for 5+ hours I finally decided it was time to pull the plug. I connected a monitor to my raspberry pi and rebooted it. The system was unable to start and I eventually had to restore from a backup. But(!) using the monitor showed several errors, the most relevant was an xhci_hcd error which lead me down a path to discovering that my decision to save a few dollars on an SSD enclosure was not worth it. Apparently this does not play nice with RP4. So, I bought this and tried again this morning. With the monitor still connected I initiated the data transfer, this time the HA text said about 10 minutes. I stayed close, watching the monitor, and in less than 10 minutes the system was starting, things were looking promising! There was one small hiccup, I saw something about a supervisor error on setup and couldn’t connect to the system (on port 8123), but I was able to connect to the HA observer (on port 4357, something new I learnt during all this) so I decided a reboot might work and pulled the plug.
Then, in a few minutes, now 14 minutes after starting the move data disk, my system was up and running!
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Thanks for your very helpful experience report 
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