Welcome to the project!
The assets of the RA and TD expansion packs got incoroprated into the basegames, while the missions (both those of the base game and the expansions) are still a work in progress, as OpenRA is (unofficially) more focused on multiplayer. Since you mentioned TS, I have to point out that a proper OpenRA version of that game is still far away, though you can download and play a functional alpha version with tons of missing features from the OpenRA Github repository.
Have fun!
Tiberium Sun isn't finished. It's playable i'm led to believe?? But is definitely not finished, and campaign missions i have no idea on.
You can play Shattered Paradise though, that's 100% playable, not finished but it's in a released and awesome state. So much so we've a few tournaments on it already. See 5aces Youtube channel for Shattered Paradise gameplay.