Funny you mention Flutter since I’ve experimented around with it a lot in the past. I loved how easy it enabled me to develop native apps for ios/android using the same codebase. If your users are mainly on ios/android and you’ve created a flutter app for them, then it would make sense to use flutter-web for the small number of your users. But I definitely wouldn’t recommend flutter-web (yet) if all or most of your users are on web. It doesn’t compare to the more mature web frameworks as it’s missing features and the performance is lacking (ex: scrolling, text selection). I really hope that changes in the future though.
As for Flutterflow I was playing around with it a few days ago to see how viable it is to use it for production. My main issue with it was that I couldn’t find anyone who has used it in production. I needed to know what to expect and how much refactoring I would need to do. To be fair though, I really liked how easy it easy it was to prototype the app design with it, which is the part I hate the most when developing.
What features in Appsmith/Budicase are missing? I’m planning to use one of them for my current project and curious to see if I missed something 
. Anyways there’s also jetadmin, they seem to highlight their support for external users/client portals.