RPGMemes

Have a read of Fate core, there's an SRD online for just the rules but the core book is great for GM advice, even if the system is way too meta for my taste now. Blades in the Dark also has some excellent GM advice and is currently my favourite game system. I'm running a Fate game currently with nothing pre written and Very minimal prep week to week.

Session zero players make characters and decide what elements they want to see in the world, vampires, Wizards, cults, werewolves. This gives me themes and factions to work with.

Between session 0 and 1 I did a fair bit of prep (like a couple hours) but only so far as main factions, key characters, goals and some aspects. Once you know who the key people are and what they want it's pretty easy to plan, they will follow their goals unless the players get in the way somehow.

Then between each session all I need to do is think how the world reacts to the last session. Usually all that means is that someone now wants to kill the PC's as top priority. My usual planning time is like 15 to 30 minutes, often less. Though I'm happy running a very improv heavy game and think I'm pretty good at coming up with interesting scenarios off the cuff. Some people benefit from having more groundwork laid out.

Regarding illusion of choice, I started like that: do you go left or right? It doesn't matter I've made one map and it's a circle so your going to the end whatever you do. I find my current way more fun, it's less work and I get to be surprised by what happens as well. Yeah it's frustrating if I have a cool idea and the players decide that the faction it translates to isn't that exciting and don't interact but given all ove done with that facton is named the leader thought about their resources and goals if it's not used yet it's not really a waste of my time.

Just to reassure you that you can run games without a module and with very minimal prep but the improv muscles need lost of practice.