What progression do you mean exactly? The katana fleet doesn't tie into imperial regimes, after Thrawn all you can do is fulfill the needed requirements for Dark empire, otherwise your progression is over.

The mission itself is a total mess too, incredibly laggy and if another faction finishes it first it just sits in your mission log, highlighting itself each time you load your game.

so after i get the katana fleet mission and complete it nothing changes, i dont get the mission to complete dark empire

Well since the katana fleet has nothing to do with dark empire, I don't know what you expected to change. Personally after completing the actual requirements (2 factions integrated, 60% legitimacy) I've had to wait a week or 2 before Palpatine decides to call me up.

hmm i might just have a bugged save or something, and have attributed it towards the katana fleet mission

You might have better luck on their discord server: link [discord.com]