Five boards is a lot for people to hold well enough to break. You mentioned spacers, I've never even seen spacers used in a break done while the boards are held. If you could get people to hold 5 boards well enough to break them, I'd imagine a spinning back kick or jump spinning back kick would be best.
Thanks for the feedback. I have seen boards put on a type of platform holder and using spacers an axe kick or stomping type kick was used. The height varied from around knee high to upper torso. I can't recall how many boards were used. I think 4 or 5. 5 and 6 boards are commonly used for side kicks at around 3rd to 5th Dan. I have difficulty coming up with something to hold more than 5 boards for a side kick. 5 boards is where my training partners and I max out using our hands to hold them in position. That sometimes is a bit unstable. Back to the main question though. Besides any type of sidekick, is there a favorite tech. that someone uses for 5 or 6 boards?
5 board break will be pretty difficult to perform ( even with kiln dried pine) and is also difficult to hold for - a spinning or stepping back side kick should do the job for you. No spacers when the boards are being held.
If you're going to go with a hand technique an overhead hammer punch would do it.