Kinda hard to give you a correct awser. Process, workflow and idea processing are always differents for each one.
Anyway you 're kinda right most of the time i think to seamless looping in a very early stage but it's not always the case.
But i don't have any general planification or something. Most of the time i'm messing into Houdini and a idea pops when i'm working on a complete different subject.
For the last serie, the steel marbles one :
At the very beginning i was asking myself how to make an efficient asset in order to create things like construction game, rollercoaster, tracks, pipe etc.
My goal was to have something you just CTRL+C CTRL+V with a type selector parameter, an orientation parameter and some auto snapping features. The idea was to have an asset who will be efficient enough to be faster than doing it in real life with a contruction game, electric toy car track etc...
After all this background work it took me something like 1 hour to create this 2 lanes track from a blank scene.
The looping was not so much a problem in this scenario. Only 2 marbles are simulated. One for the Yellow lane, one for the blue lane. i've simulated 6000 frames (very close to real time).
Then i just copystamp a point representation with a stamped time sop to loop this.
At the end i just have a 6 frame endless motion setup where i can freely put any camera.