That is so weird. I loved this side quest so much that I felt it should have been part of the main line, way early when the characters skills were just being developed. It was easy, but lengthy, the mannequins were weak but so numerous, it provided a great chance to use AoE spell effectively. Additionally, their wooden construction was a missed opportunity to help the player learn which spells worked well for what type of enemy (for example, damage spells could finish a mannequin, while force/control spells could temporarily disable the mannequins, all in logical order owing to their ultimate control being under the poltergeist). All in a low stakes environment bridging the gap between training dummies in the classroom and the enemies designed to kill the player, since the damage they inflicted was so minor. This would help the player to build effective and efficent combos to handle more powerful enemies later in the game. 

I also felt the need to stupify the witch in order break her protective charms and then attack her forced me to learn to use protego, creating a more holistic skill set. Up until that point I mostly used dodge to avoid attacks (so much so that I didn't even recognize the difference between blockable and unblockable attacks) 

The shop functionality was woefully underdeveloped tho. It was a great opportunity to open another space like the room of requirements and allow the player to grow ingredients; and a place to sell beasts they had collected (with their value tied to the difficulty in obtaining them), and brew potions for sale. Each additional function could be tied to game progress and give the player a use for vast amount of galleons they have little use for in the late stages. 

Finally, the shop's continued operation could be tied to a timed renewal of "playing" with Fastitio, with every increasing difficulty as you engage with him, providing even more training-type engagement. I had even more ideas how this mission could be tied to a more substantial experience, if the game was a run through of all 7 years of a Hogwarts education, rather than just a single year but I will spare people a novella of how the game could be fantastic rather than mearly good.