I don't know how to fix the showcase problem. With web apps, it's easy – just scan HTML, deduce what framework is used, and add it to your showcase list. With Flutter mobile/desktop apps it's not possible. And there are zero incentives for developers to send their apps for screenshots. You just ship the release and move on to the next tasks.
I also discovered Flutter in 2018, and it kind brought joy into UI programming for me. I rewrote all web apps I had with Flutter and published a bunch of mobile apps, but the coolest thing is that I routinely make apps for my own (or my team's) use.
For me, Flutter lowered the costs of UI app development so much that I could just spend a weekend making some neat, small, practical tool for my own needs. I really love making these "private" apps – no need to care about all corner cases for all users, no need to polish design for all screen sizes/font sizes. Don't even need to be released (TestFlight at max, if doing it for a team). It's just fun, and I wish more developers could have the same experience.