Animation inconsistency in anime is super common, honestly, and Frieren definitely has it. What you're seeing is the result of how TV anime production works - there's usually a lead animator or director handling key scenes while other episodes or sequences get handled by different teams or freelancers. Some episodes will have way more resources thrown at them, especially if they're covering important story beats or if a particular director really wanted to flex on a scene. Other episodes might feel more rushed because the studio's juggling multiple projects or there's a tighter deadline.
The shift you noticed around episodes 8-10 could be a few things. Sometimes studios deliberately dial back certain scenes to save budget for later arcs that matter more to the story. Or it could just be the natural rotation of different animators cycling through. Frieren's got some absolutely jaw-dropping sequences scattered throughout, which is intentional - they're putting their best work where it counts for them narratively. The "rushed" feeling in between isn't usually laziness, it's just how the whole system balances out over a season.
It's definitely not something broken with the production or a red flag. If anything, it's pretty standard for how anime gets made, especially with weekly episode schedules. Some series are way more inconsistent than others, but Frieren's generally considered well-executed overall despite the ups and downs you're noticing.