Yes, that's better, and putting the SD on read only while on the offline machine is another assurance of safety... Is an SD exploitale through hardware this way?
My understanding of SD cards is that they dont have to negotiate a utility class like USB, they are identified only as storage by the OS, and their drivers only allow for that anyways.
On the other hand, USB can negotiate several classes for the same PID&VID, mostly the infamous HID class. The grand majority of USB attacks come from that unrestricted class negotiation allowed by the standard and most of them rely on the power of the HID class.