It wasn't rushed at all. If anything, it was more focused and some of the changes made it even better. What was cut out were mostly the fan service scenes and what little filler there was. Frankly, I would have been disappointed if they tried to stretch this into 12 episodes. It would be akin to making Bakemonogatari just the starting arc with Senjougahara. Would it be good? Sure, but you'd be missing out on a lot more than you'd realize.
The final chapter here is not the end, not by a long stretch. The start of the second volume is available here and it picks up with what you'll be seeing in episode 4. https://mangadex.org/title/26207/seishun-buta-yaro-wa-petit-devil-kouhai-no-yume-wo-minai