How can a UI toolkit be more native than GTK on Linux?

You can be as native as GTK on Linux :)

GTK is not required on Linux for FreePascal GUIs.

See MSEIde, which is directly X11: https://github.com/mse-org/mseide-msegui

"MSEgui is a complete independent Free Pascal widgetset. It does not use any other widgetsets like GTK2/GTK3/Qt4/Qt5. MSEgui interacts on Unix directly with X11 server (like other widgetsets do). On Windows, it deals with the low level Windows GDI interface."

Although a separate windowing set than Lazarus, both are enabled by the FreePascal compiler.

Lazarus does have a GTK backend as an option on Linux.

as a RAD-like tool, Lazarus looks pretty good.

But I'm the author of a substantial cross-platform creative/media application, and not only do we not use any RAD tools, I don't think we could. So if you "destroy this mess", I'm not sure how we could do what we do. Very few "creative" applications use stock toolkits and RAD tools.

The RAD component is optional; you're free to code up your interface if you'd like.

Which from what I can tell would buy me essentially nothing over any other cross-platform GUI toolkit.

I don't use Qt, but QML and its associated tools look to me at least as good as Lazarus.