Leo Lichtman said:"Justín Käse" wrote" (clip) the first thing I did was to drop from a curb,
about six or eight inches and instead of posting (standing bent kneed on the
pedals) (clip)
^^^^^^^^^^^^^^^^^
I was not familiar with that term (posting) but I am very familiar with the
technique. A heavy rider allowing the rear wheel to drop off an 8 or 9"
curb, with full weight on the seat, would definitely stress the bike.
Besides punishing the rear wheel, think of the bending load this puts on the
seat post, and the prying action this then applies to the top of the seat
tube and clamp.
Mountain bikes are designed to take such loads, even though most are
never actually ridden off the pavement. I have used relatively
inexpensive frames and wheels to ride off-road for many years, often
pulling a trailer (or trailer-bike) in addition to my 230 lb body. I
have had too few broken spokes to remember, and I think most of those
had been chain nicked.
MTB wheels are very strong. If a wheel is breaking spokes it's likely
from no stress relief and under-tension. Even the cheap wheels on $250
bikes seem to take hard off-road riding well once they have been gone
over. 8-9" drops are nothing -- it's common to ride a succession of such
drops (AKA stairs), and that's one of the tamer things to do with a
mountain bike -- real trail riding is usually much more demanding.
I'm 6'10" and use a 425mm seatpost on my mountain bike. I assume the
designers realized that a long seatpost would actually be used with its
full length, and that some riders would exceed 200 lb. If not, a weight
limit would be needed. To date, I have not broken a seatpost, either --
despite bearing an additional load of 100+ lb coupled to the base of the
post (trailer/trailer-bike). I have broken one saddle -- a lightweight
titanium railed one -- cracking the rails from bouncing on the nose, but
that's about it from a decade of weekly+ MTB rides over steep and rocky
hiking trails.
Your concerns/advice are obviously based on speculation, not experience
or familiarity with the equipment. That isn't helpful.