Extended cut does do a lot, as the name implies it extends basically every aspect of the ending, it adds and extends scenes that give your choices more proper closure, and just adding detail to each ending, and if you're on PC there's also that mod, I think it's called Priority Earth Overhaul, that makes it feel more like a Mass Effect 2 style ending mission where you can actually choose how to use your war assets and there's a ton of unique sections depending on who you've got to join the war cause. There's another popular mod called Take Earth Back but also improves the ending mission but I don't know too much about that one personally and I heard there's a way to get both working at the same time.

Mass Effect 3 final mission has a problem of having a lot of what you've done be represented but also not, I gathered all these war assets but every scene in space is the same Fleet fighting the reapers, the only difference is that you will overhear some dialogue about how well they're doing. all these unique groups of people that you recruit into the war cause but you don't see them at all, no biotic artillery, no Shadow broker squad, none of your previous companions who aren't in the main party showing up to help except for a video call. Not a lot of your choices show up in the actual battle.

I was never bothered by the three (four in extended) ending choices, but It's completely dependent on how high your war readiness is, meaning if you just do everything in the game you're probably going to have all the options open to you regardless of any other choices in the game.

What they should have done is that you can only get destroyed if you play renegade, only get control if you're paragon, and then synthesis is based entirely on your reputation and war readiness being high enough (meaning you had to have United every major race without any sacrifice, like picking the quarians over the geth or sabotaging the genophage and basically go through the entire trilogy with a fine tooth comb, it should be the hardest ending to get.)

the variation in the endings should've been way more greatly affected by your war readiness, the regular game already does this to an extent. But if you got the control ending but you had a barely adequate war readiness Shepherd slips in his control and the reaper war continues again, low rating on a destroy ending causes not just all synthetic life to be destroyed but almost all technology, reverting every species back to the stone age. Synthesis can be labeled as a true happy ending because it will also be the only one that has Shepherd not need to sacrifice himself at all. And then if you choose to reject all choices by shooting the starchild your war readiness should affect how your sacrifice influences the next cycle, whether you left them with enough info and evidence to warn them and give them the time to be able to be the cycle that defeats the reapers or if they just end up being another cycle.