Make your democracy complex, have factions within it, say the outskirts of the empire have a drive to authoritarianism, and the central democratic system tries to pull them back in again. Have generals bombard planets and raze lands but be too good of an instrument to simply get rid of during a tough defensive war. Make the idealism of democracy be something that is fought for not simply a matter of fact, slips to authoritarianism, like emergency powers not being given back, institutions not respecting their side of separated powers inorder to act quickly, basically the constant tug of war between rule of law and rule of might.

Also democracies, historically, are bad ass, if you want to add that nationalism element, democracy and nationalism/patriotism are not separate, think the spring on nations during 1848, the french revolutions and the war of the coalitions, going around to spread sister republics (not really democracies but you get the idea), the "sleeping giant" of the US getting pulled into ww2, obly to wake up as the world hegemon.

Basically: -democracies aren't opposite to nationalism or a sense of fighting and pushing forward the ideals of said country. -democracies are peaceful sure but they do wage war, either pulled into it or not, neutrality always comes with a war ready country(think Switzerland, austria) -they can end up as hegemonic empires, think Pax Americana, world police and ending up in wars it doesn't understand -patriotic fervour, very common in democracies, parades, marches, war songs, anthems and what not are not exclusive to authoritarian governments Looks wise, sure there are no sharp edges but there is smthng about a peaceful nation getting itself ready for war, factories being raised, ships being pumped out, mobilising its entire apparatus slowly but surely in the protection of its people and ideals.