A tag is a direct Pin. You can not change that. You have to use branches instead, but thats a lot of overhead in terms of housekeeping the branches, especially if you want to maintain them roughly semver compatible for all three parts of the version.
Perhaps you can use a private hex repo? On a quick search I found this thread:
Also there is mix hex.repo to manage repositories.
1.3.5* as a tag specifier can not be expressed in terms of a mix version specifier. It would include 1.3.5-rc0 which is smaller in version than 1.3.5, but it would include 126.96.36.199 which is not even a valid version. Which of those would you choose as the most recent?
1.3.5* as a tag specifier would not include 1.3.6 which would be totally compatible to 1.3.5 but may contain bugfixes.
1.3.5* as a tag specifier will then contain 1.3.50, so after years of your application was stale, builds will happen to do unexpected stuff and fetching libraries which should be compatible, but god knows what happens after 45 patch releases!
I do not consider it a solution to your problem and stick to my advice of either using branches or using a private hex repository.