Whenever you run mkdir, be sure it isn’t sudo, that the new directory is owned by your regular user (check before mount, not after…you want your user owning the directory, not root). If you go one directory in from the mount point after the mount and use “touch testme” and “rm testme” and this succeeds, then it isn’t an SD card mount permission issue (you might also do this in the mount point directory if you access anything directly in that directory).
There are some cases where symbolic links are not followed for security reasons, though I don’t know if this is the case for what you’ve moved to SD (anything in a web server directory restricts this, but I doubt you’re work is linked to the web server; SElinux can also do this, but L4T does not enable SElinux). You might get logging of a security message in “dmesg --follow” or “sudo tail -f /var/log/syslog” which might hint at where the permission failure is.