yeah you have Hive-Auth and we use it. but we had to switch to Hive-Auth Mobile released by Arcange for connecting to Unity as Keychain does not give back the same results. Their are differences.
We would love to use Keychain for our Hive Unity SDK.
as far as Safari it is actually very easy to turn your chrome extension into a Safari extension you don't even have to change code.
Don't take anything as hate everyone loves Keychain but development is progressing really fast in the world it is hard for any project to keep up.