Over the years I’ve come to use timers to push the narrative forward more than anything.
PCs enter to find the location they have been walking toward is under siege by countless enemy mooks or minor enemies. maybe 1d4 more spawn every single ROUND
Roll a 1d4 timer
during this time they are trying to learn what is happening. Who is attacking? Does any NPC nearby know why? They fight and learn possible new mechanics on what the enemies do. Do they explode when they die? do they use poisons? that kind of thing.
timer hits 0…move the narrative forward
The boss enemy shows up, breaking out of the mayor’s estate holding an important item/hostage revealing their true intentions with the siege.
Roll a 1d4 timer
The boss is trying to escape with whatever they acquired and this is the time the PCs have to stop them from escaping or reclaim that which was taken. Maybe the boss has synergy with the mooks they encountered with the first timer and their knowledge they learned helps them overcome this synergy. Lesson and Exam logic.
timer hits 0…move the narrative forward
A giant dragon/bird/airship shows up to pick up the boss and the stolen thing and they escape if the Players were unable to stop them in the previous timer.
if they did stop the boss and reclaimed the stolen item/hostage then have another event resolve instead, maybe reinforcements show up for the town and drive away the mooks and victory is achieved. Something that progresses to the next scene or story event and not drag out the current one any further.
I personally write a list of narrative points using Runehammer’s Social Encounters formula
1.)where you have the vanilla “as written” story
2.)the favorable outcome, a bonus or reward
3.)negative outcome, consequence
and then you’re just reacting to whatever awesome thing your players do so they guide the story instead of you dictating it.
This is how I’ve been using timers lately and its been very effective for my group. Don’t be afraid to let your players fail. Failure isn’t just a TPK, failure is letting that boss escape with that hostage/artifact. Failure opens so many more doors to your campaign. Just my 2 cents.