Hi, I'd like suggest that the super-dreadnought tech (and all techs that depend on it) only become researchable once the ancient dreadnought that can be found on a random star system is captured and reverse-engineered.
This is more about the feel of the game than anything else, but I think it's a good idea for a few reasons:
Super-dreadnoughts look the same for all species, and so in the admittedly hypothetical situation everyone progresses to that tech level and starts building them, their ships will start looking the same. By restricting it as above, it's precursor-like look stands out more once it does appear.
If you haven't encountered the ancient dreadnought before, it's appearance (as in the look of the ship, and as in the tech tree) can be strange:
For example, as the Phidi, it's really cool how the tech tree is different than for other species when it comes to warship hulls. But then there is this super-dreadnought thing (not to mention the death star) way at the back that is presented as a standard Phidi tech.
And e.g. as Orthin, it was also strange how the super dreadnought did not resemble any of my other ship hulls until I knew about the ancient dreadnought encounter. By having the tech appear once the ancient dreadnought is salvaged, it reinforces the connection.