The Planetbound for Millennia background seems made for really old ships. However, it's noted as being available for all types of vessels. Does this include designs that are fairly recent, such as the Lathe-class, the Secutor-class, and the Tyrant-class? The Tyrant-class is specifically noted as being produced only since late M38, so it's only had - at best three millennia of existence. Is it reasonable to assume one might have spent more than half that time on a planet?

Sure; ship was kept in drydock almost since the time of its creation due to production flaws, administratum oversight, a tyranid lictor making its way onto the shipyard and killing all the crew aboard, any number of things.

Would it still make sense for such a ship to have two Archeotech systems?

Also, I didn't think that Imperial starships were constructed on the surface of planets.

As a side note, we have considered the odd idea of a Tyrant that had a warp accident and arrived at a destination WAY before it had ever departed. In fact, it arrived thousands of years before it left. After that, it came to rest on a planet until discovered by a RT dynasty that refit and relaunched it and have been operating it for centuries before the Tyrant-class was even designed. No one today would recognize the heavily modified vessel as the one lost by the Imperial Navy centuries ago - especially since it's had a recorded history with the dynasty that extends well before the time it went missing. And that's how a cruiser fell into the hands of a RT...

Well you can think of it this way: even if it wasn't planetbound for literally millenium, it could have been been a planet that had above-average gravity pull, or corrosive atmosphere, etc. Basically even though the amount of time it spent on a planet was not the same, the conditions of the planet itself accelerated the wear and tear generated on the vessel.