Tomas Slancik

you can't set inputs:file attribute on a single material primitive itself to a varying value, since that material has no idea to which primitives it may be bound and how such values would map to them

you either need a material per variation, which you can easily do even in 20.5 using Assign Material LOP and it's Parameter Override VEXpression which will create material variations for you automatically (example attached)

or the override can happen automatically using primvars during render, however for string primvars this is possible only since 21.0 for XPU

if you have to bake it in Blender anyway just to be able to export as fbx with curves, then it may be easier for you to use Bake Animation ROP to bake animation curves from Alembic camera to a new camera in Houdini, which will save you those manual steps

you can use
$FSTART and $FEND variables for global frame range
$RFSTART and $RFEND for visible frame range

so it would be something like:
fit( $FF, $FSTART, $FEND, 0, 360 )

however since I assume you want to seamlessly loop rotation, you may want to do:
fit( $FF, $FSTART, $FEND + 1, 0, 360 )

otherwise first and last frame would be the same angle since 0 == 360