Well I seem to have Juan Mendoza's R-Console bridge plugin mostly working in Qt6 in QGIS4 by replacing a whole truckload of Qt.Foo with Qt.FooThings.Foo - graphics not working but I don't think they were working on my QGIS 3 system... Code here: https://codeberg.org/browlingson/rqgis/src/branch/qgis4