First lets establish reference points for realistic weights carried by Marines.
The fighting load is roughly 55 pounds and includes combat gear necessary for the immediate mission at hand.
The assault load is roughly 70 pounds and includes gear needed to carry out an assault.
The approach march load is just more than 90 pounds and includes gear necessary for extended operations when resupply is still available.
The sustainment load is nearly 150 pounds but is intended to supply a Marine from their pack when resupply is not available. The massive weight limits mobility and distances a Marine can march.
EFT Mechanics and potential changes
The current system is fairly realistic in that ~35kg is a combat load, and ~50kg is a standard mule load, with a maximum load of 75kg.
Quick drop function for backpacks. It's fairly realistic that you would drop your rucksack to maintain combat maneuverability. I believe this feature is already being developed.
Stamina should not drain OR regenerate while walking with 50kg. This would allow you to sprint a short distance when required and continue walking afterward but you would have to stop completely to regenerate the stamina required to sprint again. This simulates the ability of Marine Corps infantry officers to carry a load of up to 152 pounds for more than nine miles, at a twenty-minute-per-mile pace. I think this would be a massive quality of life change as you would preserve your stamina in anticipation of enemy contact giving you a short burst to get to cover and drop your rucksack.
An interesting mechanic could be the first 40 lbs of gear has no movement penalty and every 10lbs of weight you add after that your movement speed decreases by a percentage. This allows people that run lighter setups more maneuverability but doesn't overly punish players for running a regular 35kg loadout. This also levels the playing field of a hatchling against someone in 40 lbs of gear as they would have the same movement speed modifier.
TL;DR
Current weights are realistic but might need slight adjustments (Linear weight penalties after the first 40lbs)
Quickdrop key for rucksacks
Weight mechanic should be changed so that you don't LOSE stamina while walking when overweight but do NOT regenerate stamina either. This gives you the ability to preserve your stamina to get to cover upon enemy contact much like you would in real life. This change is offset by the linear weight movement speed penalty described above. You can walk forever but the more weight the slower you move.