Graphnoobie If anybody has experienced this, I could use some help with a proven way to access the data on the 5a. Note: when I connect the phone to my Win 7 Desktop or my Win 10 Laptop, an Unknown Device Icon appears in the Devices and Printers Screen on both. When I unplug the phone that icon disappears.
You could try charging it for a long time to see if that brings it back, but I'm skeptical, since those symptoms sound consistent with a hardware failure. It may be the case that a repair shop or a forensic recovery company could get something off the device, but maybe not.
Graphnoobie And, since I have a Seed Vault Backup of the 5a, how best to utilize it with the newer 7a phone.
It's anybody's guess how much a modern version of SeedVault could restore from a backup made by a very-old version of SeedVault. Personally I've had too many times when SeedVault refused to restore anything at all from a backup, so I don't depend on it. Right after GrapheneOS is installed the setup wizard should offer you an opportunity to restore from a SeedVault backup. Probably it would be good to plug the USB drive in as soon as the setup wizard starts, so that the device is "seen" by the time the setup wizard offers to try a restore?
If the SeedVault backup is the sole pathway to restoring data, it might make sense to make a backup copy of the backup before trying to use it. That could be delicate, so it might make sense to have somebody sit with you to avoid some mistake being made.