I know that level impacts the attributes of a familiar when it metamorphs, but does anything else have an impact? I'm particularity curious if the treats I give a familiar have any impact.
Treats do not have an impact on stats gained after metamorphosis. The stat bonus from treats is a static bonus (up to a max of 50 stat points) and this carries over between metamorphosis.
Levelling a familiar to max level for it's current form before you metamorphose will guarantee the best possible stat growth for that given familiar.