Wounded Resihi is ineffectual
An oppressed, cowered, turmoiled, or otherwise physically or mentally wounded individual has wounded Reishi. The wounds manifest differently and significantly hamper the effectiveness of the Reishi. Not only that, the wounds reduce the length of time for which the Reishi remains effective. Also unskilled, untalented individuals contain much less Reishi than a skilled, and talented individual.
Any markets for wounded Reishi would as a consequence be literal meat grinders as the Reishi would need to be produced in large quantities, continuously. No population of humans could survive that sort of de-population. Particularly if the math works out that some 10000+ wounded individuals to 1 successful individual. That does not mean that some elements of society wouldn't try if the price of Reishi was sufficiently high/restrictive, but it would have to be very high.
The problem though is that now there are 10000+ people looking to make a quick return by literally killing the most successful people in the country. That's not going to be a fun blood bath, particularly as the people developing that technology are by definition successful individuals, even if not on the level of Olympic games athletes.
Limit the Magic
Only a handful of spells can be cast within a certain amount of time. They conflict with each other when performed too closely vigorously destroying any results, or making things worse. As such only so much Reishi needs to be collected in a given year.
Also as only so much needs to be collected, and the benefits of this magic are relatively universal society desires the most powerful Reishi to be used in their working. This Reishi is only found in individuals of truly God-like achievement.
Alternately, Reishi is perpetual. When the spell is cast the Reishi continually works at the task like a Golem. It would be an honour to sacrifice oneself to contribute one's eternal essence to works of prestige and utility, because in a sense it becomes that persons legacy. Obviously the best Reishi can produce the most magnificant works only, where as mediocre Reishi must be pooled to achieve similar feats. Thus great individuals would be more willing as their legacy is clearer, while the more mundane would not (but still might occasionally) as their legacies are less clear.
The manner in which the Reishi is harvested affects the outcome of the magic.
- Harvested just after skillful achievement the Reishi is invigourated.
- Harvested from an individual truly believing in the spell to be worked aligns it to the goal.
- Harvested from a truly well-composed individual makes the Reishi easier to work allowing more focus to be applied to the spells outcome.
- Harvested from a healthy and whole individual allows the Reishi to be stored for much longer periods of time, perhaps decades/centuries.