It is important to focus on the main issues. Since you seem to be familiar with the fundamentals, I will briefly go through the main points.
1. The “suffering” that the Buddha wanted to remove is the suffering in future lives. The root causes for that suffering is described by the Paticca Samuppada (PS) process.
2. Any future birth (jati) results from bhava (existence) grasped by upadana (the tendency to attach to vedana).
3. ALL such future births (even in the highest Deva/Brahma realms) end up with varying degrees of suffering. Death is one part of that which can NEVER be avoided in ANY birth.
Now we can see the origin of that suffering by looking at the beginning of the PS process.
4. Seeds for new bhava/jati are created by one’s mind with vinnana. There are two types of vinnana. Cakkhu vinnana, sota vinnana, etc DO NOT generate such kamma bija or seeds. It is the vinnana arising via the “sankhara paccaya vinnana” step in the PS process that creates kamma bija in javana citta. We can call this “kamma vinnana“.
5. Sankhara are our DEFILED thoughts. All three types (mano, vaci, and kaya sankhara) can lead to kamma vinnana. Our speech and actions are also based on our thoughts, and ALL are sankhara. Those that lead to future bhava and jati are strong sankhara or abhisankhara.
6. Why do generate such sankhara? We do that while we engage in “seeking pleasures” from worldly things (rupa, sadda, gandha, rasa, phottabba). But how can that lead to suffering? This is the key point that most people are unable to grasp.
– First, while we are seeking such sensory pleasures, we may engage in immoral deeds (some people think they can avoid them, but it is only a matter of time until one encounters an irresistible arammana). This is why we often hear about “good, moral people) getting caught in taking bribes, raping, or some other sexual misconduct.
– Second, as long as one has craving for “pleasures” available in kama, rupa, and arupa loka, one WILL be born in one of those. If one has craving for food (or other sense pleasures), it is not possible to avoid births in the kama loka. If one likes to enjoy rupavacara jhana, it is NOT possible to avoid birts in one of the 16 realms in rupa loka. Same for arupavacara jhana.
7. That is what we discussed in #1. As long as there is a birth (jati) in this world, there WILL BE suffering. The PS process starts with avijja and ends in “soka, parideva, dukkha, domanassa, etc. WITHOUT exception. Even if one is born in the highest Brahma realm, that life will end at some point (i.e., invariably encounter death). Furthermore, one can then be born in any lower realm including the apayas.
– Not understanding THAT is avijja.
– That is why the PS process starts with “avijja paccaya sankhara.” One generates (abhi)sankhara and engage in “pleasure-seeking” (regardless of whether they involve sensual pleasures or jhanic pleasures) because one has NOT understood that process.
We can discuss the details if you or anyone else has questions. But that is the basic framework.