Xamarin. C# is close enough to Java that you won't need to re-learn anything. Perfect for back-end devs trying to do Mobile and very friendly development environment.
I'd actually suggest Flutter for the same reason. As a fresh entrant to mobile dev, I found Xamarin apps to be peculiarly unstable, often crashing for no apparent reason.
On the plus side, though, tooling for Xamarin is a lot nicer. VS is definitely a first-class citizen, and being able to launch on a remote iOS device/simulator from inside VS is a definite advantage.
I'd second this suggestion. If you aren't already conversant in the complexities of modern JS, trying to do a web-dev-centric mobile app is going to have you beating your head against the wall and cursing a lot.
On the plus side, though, tooling for Xamarin is a lot nicer. VS is definitely a first-class citizen, and being able to launch on a remote iOS device/simulator from inside VS is a definite advantage.