That was some really interesting reading.
This looks like the ultimate decoupling:
So that'd add inductors to my shopping list... Also makes me consider amending my project to isolate the motors even more from the sensors and so. Going to be a fairly beefy inductor, 20-50 mH based on that article and I'll have to rate it at 5A or even more (and scale the 12V supply accordingly of course). Plenty of space, that's the good thing.
A solenoid I suppose does not need this kind of isolation, lights definitely not (not going to dim LEDs so no PWM), fans probably do as they're usually brushless motors again.
Definitely going to solder some caps on those motors - they may be unnecessary with all the other decoupling but also won't get in the way either. Better be safe than sorry.
(edit) that step-down is a ready made module; it comes with its own caps. I think it's safer to just use a complete module rather than trying to put that on my own board and having to worry about exact component placement and whatnot for those things.