MrKookm,
I don’t know if I understand what your issue with the returns is correctly, but just based on looking at your picture and your explanation I would say that your answer to the issue you explained lies in the tank itself, not your plumbing.
Think of an aquarium leak. If your tank cracked an inch under the top water line, the water would most likely pass through the crack in the glass and then dribble right down the glass to the floor. There isn’t enough water pressure behind it to make it squirt out. If that same crack occurred at the bottom of the tank, the water would squirt out of the crack instead of just dribbling down, because there is now 16-18” of vertical water weight pressing the water through the crack. It is the exact same crack in both situations, but the lower one will flow more water at higher pressure, because it has more water pressure from the tank water pressing down on it.
The fact that you have the returns at different heights in the tank is the main issue. At the stage you are at in the picture, the lower return has about 6” of water above it pressing down into the return, while the upper return only has 1” or so. That is 6x the force of water trying to push water into the lower return versus the upper. That is a balance that very heavily favors the lower return over the upper for the vast majority of the total return flow. As you raise the water in the aquarium that ratio of 6x slowly lessens. By the time the aquarium is totally full, there might be for instance 16” of water pressing down over the lower return on the left and 11” over the upper return on the right. Now the difference instead of being 6x is only about 30%-40%. That is why your two flows began to slowly balance out more and more as the tank filled up. It doesn’t matter what you do to the plumbing of the returns themselves under the tank, because no matter how efficient you make them that end ratio of the right return flowing about 30-40% less will never change as long as the returns remain at different heights in the tank, and when the water is low like you have it in the picture, the upper return will still always flow almost nothing compared to the lower one. If you make both the returns at the same height in the tank they will both always flow exactly the same amount of water at all tank water depths.
I don’t see anything that is necessarily any better or worse between your two returns when looking at the plumbing under the tank, at least nothing that stood out to me from the pictures.
Have a good one, Jeremy