2500k = 2,500,000? polygon ship
+3 x 1024 tx
+fixed function (~shader) on ship ->light maps

?k polygon planet
+1 x 1024 tx
+shader for planet (check user contributions)

?k polygon sky cube
+6 x 1024 tx

Nexus set to 500 in WED
Manual recommendation of ~200 for Nexus is probably dated.
not recommended:
acknex.exe -nx 4080

User Grafton suggests dropping actions, startup functions (code?), etc. ?

Another user might suggest dropping the heavy load entities first.

possibility from source (does not apply to A7):
*use very low poly model stubs, with points to indicate direction
*than morph from stub to the desired model in the action
*do not use hard-coded model strings in ent_morph function
*use (pseudo) constant strings declared in resource file (.txt) for models (sounds, etc.)
& pass those to functions
*declare them as you would in a .wdl file
*this may allow ez swapping of resources

*basically setup low res possibilities for debugging and other purposes
*plan ahead
*after a bit, building toggles might seem nearly automatic

In this case, just recall when the error message first occurred.
Your setup may be a bit excessive, but that isn't directly evident from the existing data maybe.