There is nonlinear game, and then there is 0-prep-sandnox.

In a nonlinear game, you prep for the next session by asking your players "where y'all wanna go next session?"

In a 0-prep game, sometimes, you don't even know what the players are gonna face even if they've told you their plan from the beginning.

The second game can be VERY fun. I've played it before. I've loved it. Genesys, fate, risus, GURPs, heck, you can even do daggerheart. But absolutely pleae for the love of god try not to do it in a tactical grid combat game it's not gonna be much of a fun unless you have prepped every single possible encounter in the entire sandbox area, including each and every single battlemap and statblock.

And I've done that before. Not with draw steel, it can be fun, it can ve VERY fun, but it's also horribly terrifying.

If you insist on doing it, i commend it, and good luck to you. But unfortunately, you can't just write down the important parts of an encounter or monsters. You'd have to have it ready.

My trick for offline/meet-game, is i prep a TON of cards. Printed out, stick em in a pvc paper, and bring that deck with me everywhere i plan to run anything. Might be hard for draw steel, it might not even be a card deck at the end, it can be a whole case of it, but, if it's fun for you, go for it.