"What did you not like about your last job" is a super-common interview question for which there is virtually no upside for the candidate. You can only lose points with your answer, you can't gain them.
What you --- and everyone else looking for a new gig --- should do is, bank a couple of work-process things you'd've liked to see them do differently. Maybe they should test using a different framework, or do something different with project management. Keep the answer short, objective, and anodyne.
People leave jobs all the time. It seems like a big deal now because you're going through it, but it's much less of a big deal for hiring managers. They ask that question because they think they're supposed to. When it comes up, be boring, and get past it as quickly as you can.
I would not say you left because of "culture fit". That's because (a) it's not a boring answer, (b) invites an immediate followup question that prolongs this unproductive phase of the interview, and (c) candidates are disadvantaged by "culture fit" fitting/screening. Screen a new prospective job for compatible culture after you get an offer. Try to make a prospective employer make the decision to extend an offer based as much as possible on objective stuff.
You do not need a great reason for changing jobs in this market. "I feel like I contributed everything I could contribute to that team and now I'm looking for a new challenge" is a fine reason.