I can't see any of the images (attaching them directly to the posts would allow anyone to do so; there is a thread Adding Pictures with various ways of doing this) but there have been a few failures of bikes with seatpost racks.
Sometimes it's the rack that fails. Sometimes it's the seatpost itself. Rarely it's the bike's seat tube (frame) that fails (depends on the frame design and loading).
For instance, Dogman Dan's failure was the seatpost itself, IIRC, leaving him with nowhere to sit for the ride home, while carrying the battery under one arm, riding one-handed. :/ He came up with a brace to fix that and hasn't had problems since, once the load was triangulated https://endless-sphere.com/forums/viewt ... A#p1060023
The problem with seatpost racks is that they are a long lever on a part not designed to take a leveraged load.
The heavier the load, and the farther back that load is (away from teh post), the higher the multiplication of damaging loads.
The problem is not usually with a continous high load, though this can still cause bending or cracking--it's usually the shock loads from curb hopping, potholes, etc. But even just the constant little vibration of a normal road surface is magnified greatly by a load on a lever like that.
Even a full suspension bike still places shock loads from that stuff on the rack.
So keep an eye on the bike itself as well as the rack.
FWIW, there's a lot of posts about racks, problems, and solutions, if you look around.
BTW, one problem you've probably already noted is that weight up high on the back wags the back end of hte bike around while pedalling, and makes it handle less well. So if you can move the weight down low around the axles instead, or up in the middle fo the bike, it rides better.