Actually an absolute monarchy feudal system makes a lot of sense for a vast civilization. (There are very much major problems with democracy, namely it is hard enough to get twenty people in a club of shared interests to fully agree on something, let alone a few hundred billion spanning dozens of planets.)
At that scale the emperor doesn't need to make direct decisions, he handles high level elements and delegates the actual work and details to others. The emperor is the one setting goals, others do the work to actually achieve them. The key point of the emperor is then to basically bully everyone into making sure the job gets done, and ensuring the right people are in the right places at the highest level. He becomes who the lesser people answer to when things go wrong.
Factories in one sector are under preforming, and that is impacting Imperial Economics? Emperor yells at his minster/lord/whatever in charge of that region, who in turn yells at his subordinates, who yell at their subordinates, etc, etc, and eventually the problem is fixed, or another round of yelling takes place and a few heads roll. If it continues with changes (that the emperor doesn't even really need to know about) at the lower level not having an effect, then the emperor 'steps in' and puts someone else in charge of the whole mess.