How much could the Galactic Empire mobilise for total war?

Do we ever see the Galactic Empire fully mobilised for total war?

No, in this reading of the setting. Its shattered later remnants do not demonstrate what the intact Empire could produce under an existential threat.

The premise is that the intact Galactic Empire's full wartime output has never been demonstrated. Its later shattered remnants had already suffered repeated destruction of their industry, so their performance does not settle that question. Fleet argues that an existential invasion could unlock much greater mobilisation and shipbuilding. The counterargument grants unrealised capacity to the Imperium too, citing reserve fleets and peacetime deployment patterns away from active fronts. Wartime production and full fleet mobilisation are treated as separate measures of available power.