This all sounds very cool until you need something in your app that is not provided as third party module yet, or numerous third party plugins that you are using start being unmaintained or buggy or they don't support your use case.
Then you'll need to write your own plugin in native language, but you have been writing in Flutter and you don't know enough about the native platform.
My opinion, if you don't know native development yet, make two "boring" applications in kotlin and Swift and learn the native platforms. Long term you'll be a better more complete app developer. Later if you want to try flutter or react-native you won't be locked by what plugins can do.
Dart/Flutter is the least marketable skillet that you could obtain right now in 2018.
If I'm going to take the time to learn a new skillset, I'm going to learn the one that gives me the most marketability and pays the highest that would be iOS/Swift, then Java/Android and then I might play around with Dart/Flutter.
I usually prefer to start with the fundamentals and then learn abstractions/frameworks. Take front end development for example. While frameworks change every six months, knowing the fundamentals of JavaScript/CSS/HTML is valuable long term.
The second sentence doesn't quite make sense to me. Are you saying you hire yourself out to customers as an expert in Java, .Net, C++ & web stack but with only enough detail to produce work that keeps them happy?
Yes, in a way that is it, just I haven't stated that I am an expert, rather that I know them well enough to deliver quality work.
Most companies don't have software as their core business, they just want something that works for their business case, and most of the time they even value more domain knowledge and social skills than beautiful technical solutions.
If it works, fulfils the SLAs and the internal teams are happy with the delivery acceptance review, that is all that matters.
Then you'll need to write your own plugin in native language, but you have been writing in Flutter and you don't know enough about the native platform.
My opinion, if you don't know native development yet, make two "boring" applications in kotlin and Swift and learn the native platforms. Long term you'll be a better more complete app developer. Later if you want to try flutter or react-native you won't be locked by what plugins can do.