What conduit to use for later use?
NM-B ("Romex" is a brand name...) in conduit is a pain. Conduit fill calculations treat "oval" cables like a round cable of the largest dimension, so if you need more than one you need HUGE conduit.

So, use a junction box in the attic to transition from NM-B to THHN in conduit, which makes life MUCH easier.

Properly attached rigid conduits (which can be a confusing term, since "rigid" (RMC) is the heaviest class of steel conduit, but in this case I mean "non-flexible" to include IMC, EMT and PVC) will not allow much, if any flex, since it's supposed to be attached near boxes/termination points.

So, you need a flexible product. ENT or "smurf tube" is a corrugated plastic conduit.


Flexible metallic conduit (steel or aluminum) is also reasonable. 


Liquid-tight flexible conduit is probably acceptable, but insanely expensive and something you don't need, so it's unreasonable.