Having only recently been made aware of Quakerism myself, I also came to the conclusion that it should have a more prevalent mention as a sub-current of Christian Mysticism. That association may potentially create a bridge to other similar mystical contemplative traditions, nowadays sometimes labeled under the umbrella term "nonduality", which includes Zen, Advaita, Daoism, Sufism, among others. As Meister Eckhart said, "Theologians may quarrel, but the mystics of the world speak the same language".

If you're interested in that, read up on Shakerism, a now almost-extinct branch of Quakerism that had an even stronger focus on mysticism.