I read the README and I really don't see the point. So the dev locked himself to an old 5.6 (2016), and then only has a limited subset of Qt? OK, it "includes the essential features and is easy to build from source and to integrate with an application", but why not just build the whole source package? Just compile Qt and copy the libraries you need (eg QtCore, QtNetwork, QtGui) when packaging your application. I did it in an hour with this command:

./configure \ -release \ -no-opengl \ -device linux-arm-generic-g++ \ -device-option CROSS_COMPILE=/usr/local/xcompiler/usr/bin/arm-buildroot-linux-gnueabi- \ -prefix /usr/local/qt512arm \ -opensource -confirm-license \ -make libs \ -nomake tools \ -nomake tests \ -nomake examples \ -skip qtwebengine \ -skip qt3d \ -skip qtandroidextras \ -skip qtcanvas3d \ -skip qtcharts \ -skip qtconnectivity \ -skip qtdatavis3d