Can I define a custom default run time that will be used as the default runtime on the tag, without annotation necessary on the method


If I create a custom runtime on the package.json file, then I must provide an annotation on the method(s) that I wish to invoke within the context of that runtime.

Is there a way to set a runtime that I create in my package as the default runtime, so that any annotations will not be necessary when instantiating a method–it will automatically use that custom runtime?


No, this is not a supported feature.
For python, you would have to modify py-server, which can have a lot of unintended consequences.