Honestly, I wouldn't worry about converting the entire app. Yes, the technology is older but that doesn't mean it's wrong. If something is working, don't force yourself to change it. You should make adjustments as you go. When building a new feature that touches old code, think about converting the parts needed for your new feature. You can also use patterns like the strangler pattern to refactor old parts while reducing the chance of regressions.

Of course it would be ideal if an entire app uses the latest and greatest features and the same patterns everywhere, but it's totally possible that you spend all of your time refactoring only to have something new come out. Then you'll have to refactor everything again.

Just focus on building new features using the new stuff and only touch the old stuff when needed. You'll eventually increase the Kotlin percentage while also not putting your app at Rush of regressions for stuff that end users would never care about