I tend to plan but also react. The current GM post is an example of that. On the one hand, I abandoned a plan to have you fight a three-headed dog because you very clearly don’t want to fight it. (Obviously I can’t make every monster ‘good,’ but I think there’s an interesting way to work this scene out so that none of your choices force you to fight the dog.)
And on the other hand, there was something in place from the beginning of this level which might come into play a little earlier than planned. (And
how it comes into play depends on your decision.)
I’m the same with downtime. If downtime is playing out and engaging players, I’ll gladly let downtime RP happen. If the game seems like everything is paused, I’m going to unpause the game.
I could also schedule when Level 3 will start after Level 2 is finished—like the Tuesday after, for example. And in that case, you would know downtime RP can happen without interruption. (Or you know the interruption will happen on a specific day.)

I have a setting that is intended for downtime RP. You’re moving toward it (whether you pass the corridor or take the spiral stairs).