Are your ships optimized geometry with no intersecting faces or open edges and any surface subdivision is removed from planar surfaces? That probably has a larger impact on performance than poly count due to z fighting. Also any subsystems like turrets can be cut off if they share geometry since those will be instances.
As an arbitratry rule of thumb, Unless the tris have an impact on the profile of the ship, any tri smaller than 3/the target screen resolutionship dimension is a very good candidate to be optimized away and put on a normal map (at 1920x1200 for a 1km ship 3/19201000 = roughly 1.5 meter ) now for a 10 m fighter this is around 1.5 cm so should include all panelling and grating etc. This is just a very rough limit where the pixel density and anti aliasing are going to make the geometry indistinguishable.
Set your world scale and file scale to 1 meter and then you should be exporting in in game units, you can measure ships with the movement cursor then.
Just remember that typically your fighters and for corvettes will only be around 75% of the screen at max zoom, so details like panelling are better left to normal maps. Large ships in the 1km range should probably have details that are no smaller than 2m modelled and the rest be normal maps
3k tris for a lod0 fighter/corvette
10k tri average for capitals
May be pushing 30k for ‘remasterered’ capital ships at lod0