AFAIK, it needs a combination of things
Roster set
Appropriate shipyard
Affiliation
Edits to lua files usually won't work on existing saves, so it'll probably require starting a new save.
Also make sure the build isn't initially locked and the tech level required is set to 0.

Don't forget to check your commas as well. I've missed that a couple times when editing required special structures.

I haven't messed around with SSD's, but I have done this for other unused units in the game's xml's like LAAT's and communications battlecruisers.

Another thing is steam workshop likes to auto-update mods and check their integrity fairly often, so make backups as it might be overwriting changes. Had this happen a few times.

Thank you for the response. Its confounding though. Way back in the day in the 1.0 and later 2.0 series releases it was relatively straightforward to make roster changes, but I am missing something. Darth Arbiter aka Corey Loses and others have definitely done some amazing and complex things with EAW.

Using the Assertor as an example again.
I have edited the affiliation line via addition i.e >Eriadu_Authority, Empire< or a one to one replacement for example of >Empire<

I have mimicked the required special structure format using templates of ships that are already shared across remnant rosters. >Empire_Shipyard_Level_Four | Eriadu_Shipyard_Level_Four,
AI_Category_Dummy | Dreadnought_Category_Dummy<

I know little of LUA, however I have used the Eriadu roster file as a template for modifying the Empire one. And I test only with new GC starts.

The required tech level is zero as well with initial build unlocked.

I did try something with the persistentlibrary file, but I'm really just grasping at straws at this point.

I was able to enable some Corellian ships, except the Strident crashed due to there being no model yet. Oops. The Assertor is a starting build option for Eriadu though, so as far as I can tell it is not tied to a research and date script like the Mediator cruiser is for the New Republic or to an era change like the Eclipse. I wonder what I am missing with the SSDs. Perhaps there is some legitimacy script at work.

That's a good point on the autoupdate and integrity check from Steam, but I cannot even get the changes to work once.

Best place to ask would be the discord, but the first answer basically gave all files needed.

I did more experimenting and had success. For anyone who sees this, I had to copy the entries for various ships from the unit ship xml files and paste them into their equivalent template xml files and then modify them from there.

Uh that's not needed at all actually

Maybe not for most, but this was the only way I could get ship changes to actually work in game. My other posts detailed that I made all the regular edits, but ships would not actually be buildable in GC without creating or editing entries in the template XML.