Can you go a little deeper into why the stat changes? Why lower DEX so drastically? Why lower INT and FTH? Why Raise STR above what I need to use DSK?
I lowered DEX so drastically because you don't need it for a hex build. 18/18 STR/DEX to be able to use Berseker Blade (it has the highest base damage of all katanas).
You don't gain a worth amount of damage by increasing your INT/FAI to 32, that's why I lowered them to 30/30 (the soft cap for hexes).
Regarding the other changes: more HP is always good for survivor and more stamina to be able to perform more attacks/hexes.
If I infuse my katana with dark, can I still use Dark Weapon on it?
Yes and that's where Resonant Weapon buff shines because it adds 50 dark + 50% of the weapon's dark damage. So if you go with a normal weapon you'll only get the +50 flat damage.
At 25 ATN I have 4 slots (I think... might be 5), are GRS and RW the only hexes I should equip?
You have 5 slots and potentially 2-3 more with a specific ring.
As for the spells, these 2 use 2 slots each but they will serve you well. You can of course attune other spells to test and keep some if you like them.
Just beware of the resonant spells since they consume souls for each use (500 for GRS and 2000 for RW) to be effective. Knowing that you can destroy everything so quickly with these 2 spells alone.