We actually started before this was announced, and initially it was developed for a somewhat different use case (more focusing on "recurring invites"), but since it was asked a few times, I think we can offer a good alternative with it. [2]
I understand your reasons for choosing it, but that does not change that Flutter apps feel completely _wrong_ on any platform except Android, but most especially on iOS/macOS and the web. (This is unsurprising because Flutter is essentially a modern day implementation of Swing complete with personalities, and it's just as incorrect in its styling as Swing was. It's worse for the web because Flutter explicitly eschews standard web technologies in favour of either one big canvas or lots of little canvases.)
Best of luck.
I think you're confused about how Flutter works on Android. It's not native to Android, it uses canvas with custom drawn implementations of most components there too – same as it does for iOS/macOS/web.
‡ I periodically try Android devices and bounce off them because I find the UI to be obtuse or deliberately built for dark patterns. I was helping a neighbour with his new-to-him Pixel 8a and to see the pictures he had taken with his camera on the phone, he had to sign in with a Google account — and then we disabled the backup because he didn't actually care to back up the photos (they are ephemeral for his purposes). It took 45 minutes to figure this out because the settings and controls can only be set when you have already signed into the damned account.
¶ I am not saying that the people who expect nothing from Android would find iOS any better; they have just been trained through decades of bad UI/UX in Windows and Android (because they're cheaper) to understand that they have to fight with their computing devices to get anything done, so they don't expect anything better … and is it ever delivered to them, in spades. Flutter, here, does not help — but at least it doesn't clash with the fifteen different "platform" styles on your typical Samsung Android device.