For one person building and maintaining an app to a high standard on just one platform consumes all of your time. If you start developing it on another platform the first platform will suffer.
Building a high-quality app, from scratch is indeed time consuming and generally a full-time affair for a single person. However maintaining it afterwards is significantly less effort as is replicating it to another platform. Latter is not a trivial effort by any means, but it is still much less time consuming than writing an original, because you can readily recycle (a) code (b) ideas (c) supporting infrastructure such as website, support pipeline, etc.