A Thief Rogue gets a class ability boost in Dexterity
Rogues, like all other classes, get one ability boost at character creation from their class. This ability boost is for their key ability score, which is the ability their Class DC is based off of.
Some classes (the Champion, Fighter, Monk, Ranger, and Rogue as of writing this answer) give the option to ...
The key ability for rogues says "Dexterity or other". As you noted, the "other" options come from your racket. Therefore, no matter which racket you pick you may still choose to have dexterity be your key ability and get a boost to it. The specifics of the thief racket don't change this.
However, to answer your question more directly, the thief racket ...
Thief Rogues do get a Dexterity Ability Boost
All classes, regardless of any other choice they make, get an Ability Boost in their Class (or Spell) DC Ability. The Thief gives up the option to choose Strength or Charisma (granted by the Ruffian and Scoundrel rackets respectively), and must take Dexterity for this choice.
The spice pouch requires an action, so it doesn't work with Fast Hands
The description of Heward's handy spice pouch says:
While holding the pouch, you can use an action to expend 1 of its charges, speak the name of any nonmagical food seasoning [...]
— Xanathar's Guide to Everything, p. 137
That action is a special action option given by the ...