Thanks for your reply Rookie. It's good to hear someone else click with my rationale! I feel like a 153cm camber really will complete my collection and be a great contrast.
As I mentioned, board no.3 is a 161cm stiff camber I managed to find on eBay in mint cond. for £50! I haven't had the chance to use it much yet, but anticipate it being good for piste-charging..
I love the Force, so keeping it on my new smaller camber board which will see a lot of use at park night makes sense..
And reading around, stiff, light and responsive bindings seem to be favoured for powder.. Well that would be the SL to a 'T', I think!
Plus, if anything moving to stiffer, more responsive equipment would favour both me starting to progress more; and being a heavy, powerful rider.
That makes enough sense to me to pick both those items up next week anyway - and I'll have two bindings/three boards to rotate and learn from in the Alps in December!