Why not buy one decent pen “shell” and then just buy the plastic tube with the tip and the ink afterwards?
I know many companies use pens for marketing but still, they could apply this too and also stand up for the environment while still do marketing.
Easier again? A dip pen. Doesn’t sound so easy, but you never get leaks on the paper or where you store your pen, can change ink on the fly, not limited to nib choice or handle choice by manufacturer of either.
Great fountain pen nib could cost you upward of $50 plus barrel it will go into, plus reservoir or facility for cartridges. Stellar dip nib could be $20, you can tape it to a twig, and just dip it in ink - no converter or pricey cartridges needed.