So this is all just my opinion, take as you will.

Ok, so how well do you know the era you're intending on writing in. Post War/Roaring Twenties/Jazz Era is deffinately NOT the Victorian/Edwardian Era. It may help in your writing to understand exactly what was happening around those times in order to wipe it all away. I would start with the city you intend on starting the story in. What was it like during the outbreak of the Spanish Flu? How was the population effected? What kind of measures were taken in order to stop the spread? What was the culture like? The important issues that were on people's minds before the outbreak became huge, and then just wipe it all away. Look at what happened during the Great Deppression when everybody had cars etc but couldn't afford to actually run them. They were hooked up to horses and pulled around. (In Canada they were called Bennett Buggies, can't remember what they were called in the states)

Battlestar Galactica (The latest series) when it ended was a huge controversy with some of the fans because these were highly technological people who just gave everything up and mated with early humans, and presto, they're us! It makes sense when you look at the specialized technological expertise it requires to keep technology running. I would think technology would disappear fairly quickly during a mass human extinction event.

Another thing to consider is after effects. The Stand by Stephen King pointed out that there's actually two waves of death that happens. The first is the initial outbreak that kills with the super flu/virus. The second is the people who are left behind who have bad stuff happen to them. Like one guy steps on a nail and gets tetnus and dies. A little girl is a lone survivor and falls down an abandoned well, etc.

Once you have your world completely fleshed out, you should be able to avoide the tradtional tropes of Steampunk easily.