I think there's a high chance that they are all linked. That perhaps the founders of each great religion were all "teachers at the same school", but that the teachings they brought to humanity were given according to humanity's state of evolution, similar to how students have to go through different grades. Each teacher would then teach only what that grade is ready to learn, though they have all the knowledge.

Very cool, as much as I love the wealth and richness of where I settled, I think the primary thing is the recognition of the fundamental principle of monotheism that there's only one supreme power/creator/etc and that all stems from that.

On the level down, as I study Judaism deeper and esoterically, I find concepts that are conceptually aligned to concepts I found deep in Yogic tradition.

I think the primary divine spark is a man waking up and going "there's more to this, I want to connect to it emotionally and intellectually" -- that's the starting point of religion. Whichever path you take to explore that will end up in a conceptually similar place I think (at least based on these experiences.) Good luck!

There's a strong link between the Eastern religious practices and the mystic aspects of Abrahamic religions; think gnosticism and hermeticism, or perhaps kabbalah and sufism (I haven't really studied these but they might be good avenues). Eastern Orthodox Christianity retains much of the mystic traditions, for example hesychasm is comparable to meditation.

Christ the Eternal Tao by Heiromonk Damascene was an interesting read that brings a different perspective than Westernized Christianity.

I highly recommend The Wisdom Jesus by Cynthia Bourgeolt.