You may want to consider to make the snap build upon the master branch of the upstream repository instead of the release tarball, this enables development snapshot builds that are suitable for the continuous integration for the upstream.
The appropriate snap version can be set by using the version-script key or the combination of the adopt-info key , the override-pull scriptlet, and the snapcraftctl set-version command, refer Using external metadata for more info.
BTW, it seems that you’ve made contact with the upstream, have you asked the upstream if they are willing to take over the snap’s maintenance? If it is the case then there’s no reason to transfer the snap to the snapcrafters(though both can be done simultaneously).