Original Buddhism is not syncretic.
The core Buddhism teachings (namely, the four noble truths, three characteristics, dependent origination, emptiness, six elements, here-&-now Nibbana with feeling, etc) are 100% original.
The Buddha is said to have said: "These were realities he had never heard about before".
This is the noble truth of the cessation of suffering’: thus, bhikkhus, in regard to things unheard before, there arose in me vision, knowledge, wisdom, true knowledge, and light. SN 56.11
If they were not 100% original, the whole idea of a 'Buddha' would be a falsehood since a Buddha is self-fully-enlightened without the help of a teacher (SN 6.2).
Buddhism (MN 115) states there can only be one Buddha in a world-system.
He understands: ‘It is impossible, it cannot happen that two Accomplished Ones, Fully Enlightened Ones, could arise
contemporaneously in one world-system ― there is no such possibility.’
And he understands: ‘It is possible that one Accomplished One, a Fully
Enlightened One, might arise in one world-system ― there is such a
However, lots of modern Buddhism is syncretic. Theravada is 'Maha Vihara', which refers to a Hinduistic style Buddhism from Sri Lanka (which most of the posters on this chatsite believe in).
The less profound Mahayana Buddhist teachings are very syncretic, incorporating Hindu deities and esoteric Hindu & Chinese teachings about 'non-duality' and 'non-naming'.
Judaism is also very unique in its doctrine & does not appear syncretic.
However, Christianity certainty seems Syncretic, maybe the most syncretic.
Yet the Xtian fundamentalists believe it is the most original.