Can I maintain the health of the data on a NAND USB drive with a power-only USB charger?
The latter; don't confuse flash with SRAM or DRAM; keeping it powered has no advantage to its memory retention.
Also: if you're using a higher-quality memory device, I'd expect bit errors to appear years in the future - I'd much more expect a continuously powered device to fail earlier with a higher probability for any other reason!