I do not see any reason to depend on flash drive when he can use BIP39 passphrase and backup the seed phrase just on a paper, while the passphrase also on a paper in different locations, having like two or three backups for both.
Agree. Nevertheless, If OP insists on digital backup I would advocate for industrial grade SSD card (in micro or micro format) instead of ordinary USB flash drive. Those cards have an incredible life span, are less damageable and much easier to hide (or carry in the course of relocation) than bulk drives (even of 2.5" size).