If it's a boggo USB stick (i.e. not enterprise grade flash cells) then I'd personally choose a fast, non-journalling filesystem like btrfs to minimise writes. You'll also do well to configure systemd to keep its journal files in RAM instead of writing them all constantly to the disk. See here for more info. You can also set up a tmpfs in RAM for logs and temp files (i.e. mount /tmp and /var to RAMdisk instead of the USB).