Limiting the weight on individual units and a further limitation on maximum group weight requiring you to upgrade an otherwise pointless structure with no practical use is just an unnecessary and negative feature with the game.
So remove the weight system entirely. Units would still have limits on equipment slots, so higher tier units would be better by having more slots.
For example, The Dictator has +2weapon slots and +1 component slot over the Anaconda. the Salamander has +1 armor slot, and +1 component slot over the Scorpion.
This means lower tier units could still be useful, but reasonably they would be worse than the higher tier units just do to the differences in available slots from equipment.