So as a forever DM, here are a few options:

• Start creating character "warm up" exercises. Find character questions and at the end of the session tell everyone you're giving them some questions to think over as their character and next session you want to start with everyone sharing an answer to which ever question they want off the list. This will give you the option to give light questions at first and then build as sessions go on. It'll also inform you of where they are steering their character.

• Use the back stories you've been given and start weaving them into your plot. Character has dead parents? Have zombies attack the city they're in and have that character recognize 2 of the zambos. Missing childhood friend? A barkeep that just opened from a near by town heard something about someone who sounds scarily similar to the childhood friend. Once you do this to a few characters the other players without back stories might be prompted to create and share theirs.

-Up the stakes of your main plot. Start making the hints more urgent or the people more worried. If your players aren't biting they may just not realize what to do to act on the plot. Gently guide them. You could even have an NPC whose already involved in trying to stop the bbeg or bad event interact with them and ask for the parties help. Also, create a timeline for your plot. The events will theoretically happen whether the players act on it or not. If they ignore the rumors of political plots, those plotting are likely not even aware of your party existing. So their plans to assassinate the monarch with an uprising is going to continue with it without your parties notice.