If LEP was based on percentage of colonisable planets in the entire galaxy controlled instead of number of planets controlled it would work much better.
It would scale much better. It wouldn't work much better though. You'd still be incentivised to expand as rapidly as possible, and then arbitrarily punished beyond a certain point - this just changes when the mechanical problem kicks in, rather than solving the mechanical problem itself.
Of course negetive approval actually has to matter enough too. But neither of those is inherently difficult or hard. It's all about how you set it up.
While neither are hard to implement, they are inherently hard to balance. Approval cannot wear two hats.
Increasing the power of negative approval actually encourages expansion. The hardest reduction in approval comes from the point where it is divided by population. That will always be harder on 'tall' empires than 'wide' ones, because you get 'free' approval from the colony base - which means more total population without needing approval buildings, which means more free production. It's easier to keep 100% approval with any given population size over 3 on multiple worlds than on one world, even with LEP:
4 pop on 1 world = 70% approval
4 pop on 2 worlds = 100% approval.
4 pop on 3 worlds = 100% approval.
4 pop on 4 worlds = 100% approval.
5 pop on 1 world = 56% approval.
5 pop on 2 worlds = 100% approval.
5 pop on 3 worlds = 100% approval.
5 pop on 4 worlds = 100% approval.
6 pop on 1 world = 47% approval
6 pop on 2 worlds = 87% approval
6 pop on 3 worlds = 100% approval
6 pop on 4 worlds = 100% approval.
7 pop on 1 planet = 40% approval
7 pop on 2 planets = 74% approval
7 pop on 3 planets = 100% approval.
7 pop on 4 planets = 100% approval
I think you get the picture. THIS is why approval is a poor choice of mechanic for limiting big empires, as long as it is also being used as a brake on the development of individual planets. The problem with Gal Civ presently is that there's lots of limiters on upward development (maintenance, approval, the fact you actually have to wait around for it to grow), there's almost none on outwards-development, and the few that exist actually punish 'tall' empires more.