If you're looking for an isekai set in a fantasy world with a heavy focus on action, look no further than Lock & Key: Resonance. With the author's superb prose, it's real easy to get invested in the story early on. Things might be a bit generic and tropey at times, but in terms of isekai, it can easily nestle itself in there with other series that have gone on to get anime adaptations. Definitely give this a read if you're a fan of the author and/or isekai.