OP can connect the Kickr to the Wahoo Fitness mobile app and put the trainer in Sim mode at 5% slope with body weight correctly set in the app to isolate whether the trainer resistance feeling too hard is due to Rouvy (which I doubt).

I've periodically seen complaints by people about how Rouvy has too much resistance but usually it's by people who're used to Zwift's default 50% trainer difficulty that halves gradients actually sent to the trainer.

Is the 4iii powermeter single-sided?