ruamel yaml cloning a node without merging anchors

I have a YAML file with a similar structure:

main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test

What I'm trying to achieve is cloning the node dev by performing a deep copy of all its properties - including the merge operator - and changing its name to stage.
The desired result is the following:

main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test

stage:
  <<: *base
  key3: test

I've tried doing this using ruamel.yaml library by running the following code snippet:

import copy
import sys

import ruamel.yaml


yaml_str = """\
main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test

"""

def main():
    yaml = ruamel.yaml.YAML(typ='rt')
    original_yaml = yaml.load(yaml_str)

    copied_stanza = copy.deepcopy(original_yaml['dev'])

    original_yaml['stage'] = copied_stanza

    yaml.dump(original_yaml, sys.stdout)


if __name__ == "__main__":
    main()

However I get the following result:

main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test

stage:
  <<:
    key1: test
    key2: test

  key3: test

  key1: test
  key2: test

which is different from what I want since it's flattening the merge operator in a very weird way.

I believe the issue is the way I'm copying the node, however I've tried searching the web for hours and tried a plethora of different approaches without any luck.

Any advice?

UPDATE

After Anthon's answer, I was able to get closer to the desired result.
However if the node that I'm trying to copy has one more level of depth (see dev.key4 in this example):

main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test
  key4:
    subkey1: test

then the generated yaml will start creating anchors and aliases for all those nested nodes:

main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test
  key4: &id001
    subkey1: test

stage:
  <<: *base
  key3: test
  key4: *id001

How can I avoid the creation of those?

UPDATE 2

I adapted the solution provided by Anthon to handle any level of depth using a recursive function:

import sys

import ruamel.yaml


yaml_str = """\
main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test
  key4:
    subkey1: test

"""


def create_copied_stanza(dev):
    copied_stanza = ruamel.yaml.comments.CommentedMap()
    for k, v in dev.items():
        if k in dev._ok:
            if isinstance(v, ruamel.yaml.comments.CommentedMap):  # Add your condition here
                copied_stanza[k] = create_copied_stanza(v)
            else:
                copied_stanza[k] = v
    return copied_stanza

def main():
    yaml = ruamel.yaml.YAML(typ='rt')
    original_yaml = yaml.load(yaml_str)
    dev = original_yaml['dev']
    copied_stanza = create_copied_stanza(dev)
    copied_stanza._yaml_merge = dev._yaml_merge
    original_yaml['stage'] = copied_stanza
    yaml.dump(original_yaml, sys.stdout)


if __name__ == "__main__":
    main()

The mappings in your YAML are loaded as ruamel.yaml.comments.CommentedMap instances and they
have __deepcopy__ implemented just so as to preserve the merge key.

To get what you want you currently need to copy the keys owned by the
mapping and the attribute that contains the merge information.
Make sure to pin your ruamel.yaml version as these internals might change
in the future.

import sys

import ruamel.yaml


yaml_str = """\
main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test

"""

yaml = ruamel.yaml.YAML(typ='rt')
original_yaml = yaml.load(yaml_str)
dev = original_yaml['dev']
copied_stanza = ruamel.yaml.comments.CommentedMap({k: v for k, v in dev.items() if k in dev._ok})
copied_stanza._yaml_merge = dev._yaml_merge
original_yaml['stage'] = copied_stanza
yaml.dump(original_yaml, sys.stdout)

which gives:

main: &base
  key1: test
  key2: test

dev:
  <<: *base
  key3: test

stage:
  <<: *base
  key3: test

If your merge key is not at the root level, you have to make a function that does this for you, recursing in
any mappings,
but for your YAML this is not necessary.