Coastal batteries are a great idea to consider. Trying to shot ships in orbit through the entire atmosphere seems quite challenging enough though, and even if you have massive railguns that could do it, travel time for the projectiles might make it impossible to hit anything that isn't just coasting at a constant speed and velocity.
Orbital platforms might be more feasible, though. Since they are meant to always stay in a low orbit, you could make them very heavily armored and equip them with extremely strong weapons that would be impractical to try to move around on a ship. You would need a couple of them to always have one above a specific location in a planet, but generally the idea seems to have some merit.
I've never been able to make up my mind about missiles. Since a missile can steer itself into a target after it has been fired, it certainly makes things much easier to hit at anything but really close range. But at the same time, tracking an incoming missile in space is trivial. and once it gets close, railguns could shot them down with almost no travel time between firing and impact. Defensive missiles would work to.
The one way I see to get past such missile defenses would be to launch a large number of missiles at once, but I don't want to turn this into Macross.
The reason there are currently a dozen species with TL4 and 3 is because one or two of them developed it and then brought their tech to other worlds they discovered.
While the time period that inspired the setting certainly had a battleship craze and thought the concept was the greatest idea ever, in practice things played out rather different than expected in the world wars. Battleships were designed to be able to take out other battleships, which were the only things that could stop them from tearing through everything else that they encountered out on the seas. The Battle of Jutland was the only time large numbers of battleships engaged each other, and the experience led to all navies trying to avoid having their battleships getting anywhere near other battleships. A somewhat controversial position, but I think the people who say the whole battleship concept was a failure are mostly right. Battleships don't exist anymore, and with modern weaponry (which really doesn't have anything to do with this setting as established above) even cruisers have become impracticably big and are now very rare. In the end, a major factor in battleships becoming obsolete while they were still being build was airplanes. And I admit that in a setting without starfighters, the existence of massive battleships could be more plausible.
But I see the possibility of space battles only making sense with relatively small scale engagements, and the whole military and diplomatic framework for the setting is based on that assumption.
In the end, it's a mashup of ideas that come from my considerations of what space warefare could realistically look like, and obsolete influences for aesthetic reasons. I am constantly combining things that I regard as necessary because anything else would be unrealistic, with things that are arbitrary because they evoke a feel that I like. I guess all space settings do that.