You mention "standing on one knee", do you mean kneeling? Generally, I would discourage you from contact with the bottom at all, as there is always the risk of environmental damage.
There are some places in the world where kneeing is "allowed" and that is on sandy patches where you may watch shark feeding in the carribbean for example.
Sandy bottom sems to be dead, but this is completely wrong and damage to marine life (or you kneeing on a Weever orsimilar) can still happen!
Actually, I don't just kneel or stand anywhere. This is always during instruction sessions and always on sand or dead rock/rubble. Never anywhere where it could potentially do harm to aquatic life. I'm very conscious about aquatic life when I'm diving, which is why I flag this up as an issue that I'm hoping to address with new lightweight but slightly negatively buoyant fins. (if I can't fin backward, that introduces more risk).
I also did get my PPB, but I got that before I got my own gear. The 2 PPB dives do not teach a diver sufficient trimming or buoyancy effects of gear and its use.
Carrying weight requires the addition of more air to achieve neutral buoyancy. This further adds to potential buoyancy shifts as the air moves around the buoyancy system. It is interesting that you record that with steel doubles, the situation is exacerbated-this is perhaps due to the fact that they are very negatively buoyant and hence you are adding significantly more air to offset this? The reason that ankle weights failed to prevent the issue may well have been due to the fact that you added more weight, requiring more air and hence more instability...
This is an interesting point that I may try out. I don't use a dry or wet suit. I only dive with a rash guard and shorts as it's usually 28 degC in the water.
However, I did get the feeling that I could do with less weight (e.g. at the end of the dive I still have some air in my BCD to keep me neutral, and that's with a 15l tank). Currently I'm on 4kg. I'll try to reduce that to 3 and see if my trim gets better as a result.
As a note, a camera can definitely affect buoyancy and trim. If the camera is 1kg negatively buoyant then thats extra weight. I actually used that to trim my position with my previous BCD. If i wanted to be more head down I pushed out my camera, and if I wanted to be more feet down I pulled it in. With my current camera rig, BCD and fins I'm nicely horizontal, except my feet want to go up.
So I'll try out that theory about less weight and less air before changing more gear...