Hi @kelk, welcome to the Elastic community forums!
Currently Filebeat only allows external configuration files for modules and inputs. Moving your output configuration to an external configuration file is not supported. Feel free to file an issue for this enhancement request here: https://github.com/elastic/beats/issues/new?template=feature-request.md.
One thing I'll mention is that Filebeat (or really any Beat) can only have a single output. By contrast, Filebeat can have multiple inputs and/or modules. So moving those to external configuration files might make more sense than for a single output. But this is more just an opinion; I think if you can make a strong enough case for this in your enhancement request issue and there's interest from other users in the community as well, we'd definitely prioritize it!
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