If you look in the Lazarus source, lcl->interfaces->qt5 there is a directory in there with everything ypu need, including instructions on building.
I am a bit surprised that you say you must have that newer version to use the release candidate. Can you tell us what OS you are using ? The only currently supported (mainstream) one that might be an issue is U18.04. U16.04 does not have a usable QT. U18.04 does need a new libqt5pas but should not need the 'newest' one on that github page, that newest incorporated some changes Zelijko introduced, and then reversed, that supported a screenreader.
Davo