This is moreso about the (human-intended) tools, data, and environments you have available to you. Wanna do defense? Get more telemetry. Wanna do red? Get solid test-bed environments. Mature infosec programs are benefiting the most, good-guy-side wise, at the moment; because they've got these things in order already.

As far as harness engineering goes, it boils down to your ability to clearly define goals or success criteria, and safely facilitate the necessary access via the harness. There is no easy single piece of advice here, sadly. Though it would be helpful if you said what 'for Cyber-security ... other user-cases' means in your case.

DARPA’s AI Cyber Challenge (AIxCC): Competition Design, Architectures, and Lessons Learned: https://arxiv.org/html/2602.07666v2