This question is probably more appropriate for #multiplatform, but in general it should be easy enough to gradually adopt KMP for your Android app, but it would be a bit more difficult with the website. Kotlin in the browser is much easier to run as a Single-Page App with Compose, and I don’t even know if it would be possible to nativly integrate that with the Next.JS app.
The only way I would really see if working is to have the KMP app replace pages from the NextJS site one at a time, and use your load balancer to direct traffic to the NextJS or KMP site based on the URL path, gradually moving more URLs over to KMP until there’s nothing sent to the NextJS site anymore.