The cloaking device wasn’t originally part of the design - they borrowed it from the Romulans for use in the Gamma Quadrant exclusively.
The idea of a supremely over-powered and over-weaponed “little ship” does make sense as it can fly circles around the cube and let loose on it but it really only makes sense to me if they planned on making multiples and just bombarding the Borg ships. They were even designed for a skeleton crew so this tracks if they’re considering loss of life. The small crew that runs a Defiant class versus the full crew compliment of a Galaxy, for instance, is a massive difference.
I believe it’s even mentioned that they scrapped building more Defiants as the prototype was bursting apart at the seems which is why they let Sisko take it. Only O’Brien was able to figure out how to stabilize it and thus more were built after.