That's very nice work !!!
I would leave the barrel finial on.
When you ask the question: "Will the pen look better with a 5 mm brown centre band.." it seems you might be thinking of cutting the cap at the join and inserting a 5 mm brown ring there. Or perhaps you are thinking of an actual centre band at the bottom of the cap.
Personally, I do not think such a centre bad will compensate for the join.
It seems you are stuck with the join, unless you cut the cap at the join and insert a brown ring (I would make it about 3 mm) as an accent feature. Possibly do the same at about the middle of the barrel. . Even two thin rings (separated by an orange ring of same thickness) on both cap and barrel ... would help you get over the spacing proprotions.
But don't remove the barrel finial ...
That's my opinion.