Given your "system under test", we are building automation to test it, vigorously and thoroughly. This effort, to build a test platform, will be a significant effort. What keeps the effort moving forward and on track?
The Japanese have a saying: "You can't automate some activity well that you can't do well by hand".
That doesn't mean that everything must be done by hand but instead, successive iterations of the automation can be created in order to gain experience and to chart the course of the eventual implementation.
This introduces the second suggestion: seek to locate defects with the test platform as soon as possible. Work with the developers to understand what can be tested and what they need to debug an issue. You can't go wrong producing a defect with all the collateral needed by the developers to debug the problem. And note that even if a feature isn't complete yet, it may be productive to start developing the automation for it.