Zero does not work anymore - in newer (starting 6.4.something, may be earlier) 0 is auto converted to 1. So you can't ensure that some interface will not be used while others are up inside SDWAN. Their usage may be close to 0 of course, but not absolute zero. And additionally, you will always have Implicit SD-WAN rule which includes ALL interfaces as possible candidates for traffic.
Without much digging I guess, as you already mentioned, you can exclude such interface from SDWAN completely and set its route priority so it will appear in RIB only after ALL SD-WAN members are down.