Best solution, IMO, is the iFlash Quad at iflash.xyz
I don’t know if this unit can take 512GB micro SD cards yet since they’re a bit expensive at the moment, but 400GB micro SD cards work just fine in it and use very little power. My unit with 3000mAh battery gets charged only once per month and is never low on power when I charge it. Performance is excellent on it too. To fill up my 1.6TB iPod, it does take about 16 hours, but subsequent syncs take maybe 2 minutes to download the database file from the iPod (which is in itself 1.5GB in size). After that, it goes fast.