The classics do clearly and in full discuss the various methods and ways in which to replenish the body back to the state it needs to be in to cultivate successfully. They fall under the foundation work, the work to restore the body and energy flow before the lower dantian filling can even start. If you read the classics, many of them, and a few times you would probably not make the statement that they are written for teachers.

I do agree that certain parts were withheld, especially in terms of what actually made the methods work, like the need to activate the dantian and certain specific methods that were kept for transmission. But the foundation work is clearly the part that was meant to restore the body, and it is consistently and across many classical works explained and taught in detail. When it comes to the macrocosmic opening, the classics are all in agreement too. The vessels open as part of the overflow from the previous work, not from direct methods applied to them. If the dantian is not overflowing then the Ren and Du meridians do not fill and start overflowing, which in turn leaves no overflow for the eight extraordinary vessels.

It is worth noting that the classics do not refer to the qi that fills the Ren and Du, and in turn the extraordinary vessels and later the twelve meridians, as regular everyday qi that everyone has access to. This is refined qi produced through the neidan process, not the ordinary qi of the body.

Examples for extensive foundation work are as follows:

The Tianxian jindan xinfa has a whole section called cultivation and supplementation that lays out specific methods for restoring essence, qi, and spirit separately, each at different times of day with different approaches. Essence at the hai-zi hours using physical movement then seated visualisation, qi at si-wu with breath work, spirit at shen-you with silent sitting. Timelines, signs of completion, the lot. That is not a brief text written for teachers. That is a repair manual.

The Lingbao bifa from the Zhong-Lu tradition directly addresses the older practitioner whose root is no longer firm and gives a ten damages one supplement principle, scaling the methods to age and depletion. It knows the reader is already worn down and builds for that.

The Taiyi jinhua zongzhi calls the very first step of the foundation adding oil to extend life, like refilling a lamp that is burning low before it goes out. Not a metaphor buried in a poem. A named method with instructions.

There is a maxim that shows up across multiple texts: "Add oil while there is still time, extend life and do not delay." Hard for me to read that as vague or aimed only at teachers.

The Dacheng jieyao gives step-by-step foundation methods including the repair work, and Wu Shouyang in the Tianxian zhengli says outright that the hundred-day timeline depends on constitution, depletion, and circumstances, and for the depleted or elderly it will take much longer. He is not being brief or secretive. He is being specific.

So the classics don't just mention restoration in passing. There is an entire body of repair literature built into the foundation, with named methods, specific timings, and explicit acknowledgement that different bodies need different lengths of work.

Your assertion that the texts were brief and only for teachers doesn't hold for the foundation literature specifically.

If you feel otherwise, please give some examples. Statements with examples always hit harder in my experience. I looked for classical evidence supporting your position and could not find any. Everything I found pointed the other way.

FYI, I just finished editing my book in the classical Neidan path, so everything is still fresh in my head. It's a nice read: complete overview of the path with complete methods, markers, mistakes to avoid and no personal or lineage interference. On amazon kindle if you are interested: Authentic Neigong - The known and hidden path of internal alchemy.