I'm just a bit concerned how Flutter is basically going its own way rather than building on web standards. E.g. HTML provides declarative layout while JavScript is ubiquitous and could also be transpiled to native code. Rather, Flutter is building its own declarative UI approach, rendered with its own canvas, building on a significantly less ubiquitous language/ecosystem, and is principally governed by a single organization.

That said, I am seriously considering using Flutter for a future development project, but am on the fence due to the value and familiarity I see in the web platform.

I wouldn't use Flutter for web development. It solves targeting iOS and Android without having to use a Javascript Bridge. Neither mobile platform defines its interfaces in terms of HTML. There are definitely a few development teams where React Native is also a valid choice, but cross-platform mobile app development has a learning curve regardless.