We've just released OpenAPI Generator v6.0.1, which includes lot of enhancements in the R client generator. Please give it a try and let us know if you've any feedback by opening an issue in the Github repo.
Here are 3 simple steps to generate a R client for the Petstore API:
Rename it as openapi-generator-cli.jar
java -jar openapi-generator-cli.jar generate -g r -i https://raw.githubusercontent.com/OpenAPITools/openapi-generator/v6.0.1/modules/openapi-generator/src/test/resources/3_0/petstore.yaml -o /tmp/r-petstore-client/