I am glad that you are getting the basic idea.
I just want to say a word about your statement: “The seed and tree analogy helped further. I have a son, and I can see that his parents, grandparents, and all our combined karmas contributed to his cause of existence.”
That last part is not quite correct. That may be because you may have gotten a somewhat wrong impression from my “tree analogy.”
In the case of your son, his existence is based MOSTLY on his own kamma.
– You provided the “chemical base” (zygote) for your son’s gandhabba to build a new human body.
– In the same way, your parents provided the chemical base for your gandhabba to build a new human body.
(The zygote’s role is explained in the post, “Buddhist Explanations of Conception, Abortion, and Contraception“)
Now, when you are reborn a human again (assuming that your “human gandhabba” has more kammic energy left) another set of parents will provide the chemical base (zygote) to build a new human body.
– But at the end of the human bhava, you will transition into a different bhava, say a Deva bhava. That is when your gandhabba will make a “bigger transition.” That new Deva is not the same as the human gandhabba but without the continuation of the “human gandhabba lifestream” that Deva would not be born. That is what I tried to explain with the “tree analogy.”
– Sometimes the analogies are not as good as we hope. My apologies if it led to a misunderstanding.