Hi, Gwiwer. I'm not at all an expert, being fairly new myself. Here are just some thoughts I had while reading your eloquent post. I do get to the point eventually
The teachings on impermanence, karma, etc. are geared for our relativistic viewpoints, as beings in samsara. So the idea "something in me existed in the past, exists now, and will continue to exist; it is acted on by external forces" is in line with our apparent experience, and a good foothold. The teachings start with this experience to hook us. I would be cautious though in stopping there, because the Dharma isn't done reeling you in yet
Here is my view of things: karma brings attention to the nature of our situation and the consequences of our actions. So we diligently establish ourselves in the present. Impermanence makes us scrutinize this experience and our reactions. The present becomes even more ephemeral and unattainable. Perhaps practicing in this way, we feel acted on by forces or that we are acting on forces. So the same focus is applied to this subject-object relationship: our "this" being changed by "that." Where do we cleanly cut one thing off from the other and tie them together with a thread of intangible action? Let alone when we're talking about representations of patterns of ideas of perception... oy! The whole thing may start to seem like a weird daydream, especially with something as abstract as, "I improve myself with Buddhism" (or my consciousness). When it feels bewildering enough, the Buddha just shrugs at you. So for want of anything better, you drop the whole thing. Then... wait. What was that?
So my point with all this nonsense is that all the tools of Dharma converge at one point: reducing self-cherishing, no longer imputing a self from smoke and pretty lights. In our case, from flesh and the appearance of phenomena. The rest is just method. What happens after you get to that point? Hmm...
Having at least this entire lifetime practicing the rather astonishing ability of creating a new self out of essentially nothing, we are masters at doing it with anything, including the Dharma. I see a slight tendency towards that in your language of a thing being molded, conditioned, disciplined, especially a thing that not only endures, but that can be improved upon with spirituality. That leads to...
To me, the purpose of Buddhist practices are to maintain the thoughts, actions, physical and mental conditioning, discipline, and karma necessary for taking control of this consciousness and shaping it into what you wish it to be rather than allowing natural forces to shape it for you, often in very negative ways.
I don't quite agree
Accumulating merit or beneficial karma, which a lot of the practices help you do, is for the purpose of giving it away
It's not to feel we can make something into something else, or gather favorable conditions around us. Why accumulate something to give it away? Because especially if we feel it's of great use to us, then by giving that precious jewel to beings sorely in need of help, it helps us practice reducing self-cherishing, or bodhicitta by another name. All Dharma agrees at one point, remember!
So exercise great caution in any spiritual pursuit. We don't usually turn things to our own advantage out of some devious malevolence. But consider that you are here with us in relativistic samsara, so there is already a well-practiced notion of self. We can screw things up just out of sheer force of habit, like a guy who pulls into work then goes, "Shit, I meant to go to the store." I say this out of a desire to help you avoid something I run up against all the time
When I feel I'm being very spiritual or that things are going smoothly, I have to make a point of asking. "Why am I doing this?" If the answer isn't instantly "for the benefit of all beings" in a sincere way, then it's likely spiritual materialism hiding in the wings. So we just have to offer up the merit of recognizing our own foibles and renew the aspiration of bodhicitta. You'll likely know when you're BS-ing yourself.
To be honest, I often find myself getting discouraged and wondering if I'm practicing the wrong religion because, when I read Tibetan, Zen, and Theravada scriptures, they seem mostly in accordance with my beliefs, yet, when I read articles by many modern Buddhists, scholars, and self-described experts, they often seem to contradict many of the things I believe.
Throw your beliefs out the window and practice diligently. From your post, I feel you and I are similar in this way. I dithered between those three very schools you mentioned. Zen, so simple and direct. Theravada, the Buddha's own words. Tibetan, using our nonsense as the path itself. I have a subtle desire still to get it "right". I can't be following the wrong religion, the wrong practices. I can't waste my time.
This line of thinking is, unfortunately, egocentric. You aren't here to pass a test. Often for me, needing correct knowledge was a gateway to some practice, but more of a hindrance to much practice.
This is often our introduction to the Dharma, a logical thing that we put to use with modest success. We see how karma is at work in our lives, we can't really argue against impermanence. Seems logical. Or even we have the idea of ghosts and reincarnation and put a "factual" sticker on it.
Does it reduce self-cherishing and grow bodhicitta? Ask yourself this relentlessly. Be willing to do something wrong and useless. Show me the color of a thought that is right and one that is wrong, and we'll have an interesting conversation if we even get that far. Zen, Tibetan, Theravada, Mahayana, Pure Land... different colors, who cares? Show me your ease in discomfort, your compassion to those who despise you, your equanimity as your life and body dry up then shrivel to forgotten husks. Most importantly, show me the weight of your self. Is it a grain of salt or an elephant? It's a weird request, but if you're anything like me, Buddhism can become another mental circlejerk while your precious, short life flies by with alacrity, and your own suffering sits on the shelf along with that of every other being.
What settled me on Tibetan Buddhism was simple
I applied different practices and felt those of Tibetan Buddhism, such as tonglen, lojong, saying manis, had efficacious results. And a Kagyu center happens to be 30 minutes away. Your own karma will predispose you to one or another practice, so trust your gut feelings on this one. It's all part of the path, dithering and doubt included
I hope this wasn't too brash or seemingly dismissive. I love reading complex philosophical articles and untying mental knots... it's all so fascinating! But for goodness sake, if any of this long response resonated, get out of your head and just practice! You may die at any moment.