Thank you, I ended up using the new emuMMC.ini (with sectors 0x2 and id 0x0000) and changing the paths to ER00 and it worked!
For future reference, I:
Used NxNandManager to get FULLNAND.bin from the 256GB card and backed that up, copied all SD card files to an external drive
Erased the 512GB card with MiniTool Partition Wizard, then created a new FAT32-formatted partition (as per
Rentry’s guide)
Opened NxNandManager, clicked “Tools” then “Emunand Creation Tool”. Made sure “SD partition” was selected, clicked “Select target disk” and chose the 512GB SD card, then clicked “Create emuNAND”
When finished, went to the backed-up SD card files from 256GB card. Copied them to 512GB SD card. “Yes” if asked to merge, “No” if asked to overwrite.
When done, moved files from RAW1 to ER00. Changed emummc.ini to path=emuMMC/ER00 and nintendo_path=emuMMC/ER00/Nintendo
Plugged in jig, plugged Switch into my phone with the Rekado app while holding Vol up, booted using Hekate 5.8.0. Launch Atmosphere, the logo flashes a few times, and the Switch boots with all my data is intact.
Hope that helps someone in the future.