Need advice

I'm trying to make a compact bookmark fountain pen. However, as this is my first time getting something printed, I'm not too sure whether the design would be errorproof. Can you guys have a look at it and suggest changes please? Link here: https://www.tinkercad.com/things/fMm48ZqIio8/edit?sharecode=n3MkBIDkVY\_tyjho\_X0pE73awQWLvEcsl1SWU1p\_FuQ(on tinkercad for ease of sharing)

My primary worry for the design is the top bar that keeps the nib in place ( sagging issues during printing )

I am also worried about expansion and contraction, as the cartridges are friction fit. I hope to make a titanium version in the future. Thanks!