For the first part there are only like, one or two classes of ships that actually mount effective point defences.
Later you will get more durable fighters, enemy fighters get better weapons, and enemy capships mount more effective weapons like AAA beams~
If you need a bit more durability, lower the difficulty a bit.
I made it so that the AI should technically behave the same across all difficulties, and the only advantages you get at lower difficulties is AutoMagic damage reduction.
I tried to keep a manageable difficulty curve, but it's going to be hit or miss depending on the person. Can't make it perfect for everyone! (though if you find it too easy, turn up the difficulty, and if it is too hard turn it down!) That is my best advice.