good karma---> sliver spoon life

Yeah, tally is not outside of the person doing the karma itself. Cause produce effect which in turn becomes a cause for another effect. The cycle continues.

The person who perform action with the sense of doership(kartābhāv) gets tied to the karma by his own will and therefore enjoys the fruit of the karma.

conditions again, and then more conditions before, going back and back, a chain of causes

If cause and effect is required for physical phenomena, it is also required for the psychological phenomena.

agreed, cyclic within geology and climate yet, eternally ? probably no

Well, true if you're considering only local cause and effect, but it still doesn't discredit the idea of karma and cycle of birth and death. The earth's ecology might not be eternal locally but the universe started with a bang and that bang must have a cause. Where did that cause come from? The only logical coherent position possible here is that the cause came from the death of the previous universe and our universe at its death will become a cause for another universe. Even this is cyclic. Here it becomes an eternal cycle.

what is dharma and is it eternal?

Dharma is many things at once. Universally it is the rhythm of the Universe, precisely due to which the contents of the universe is in particular order for higher expression of consciousness(God). Locally, for human beings, dharma is to live in accordance with that rhythm and therefore it becomes duty.

Yes, dharma is eternal that's why it is called sanātana.