Not so much gameplay tips but strategy advice:
1) Don't forget about the niche units.
Missile Frigates ruining your day? The AI unlocked a bonus ship that fires Missiles? Unlock those counter-missile turrets and laugh at them.
That resource node next to a hostile warp point and being destroyed every 2 minutes? Get the Harvest Shields and upgrade it so it does not get destroyed again.
It is worth you time to take 5 minutes, pause the game and just read through everything you can build. There is nothing 'hidden', everything you can build shows in the build menus, it is just grayed out until you research them.
2) You've only lost once you lose your Home Command Station.
Several of my early games I would end up losing half my empire to the AI but my mobile fleet would catch up and destroy the AIs forces before they could finish my Home Command station off. I would then go "I can't rebuild this in time" and start a new game.
Now I know better, after killing off an AI's attack you (usually) have 5-10 minutes of free time to rebuild which gives you quite a large window to recover and go on to win the game.
In other words, everything is expendable no matter what it costs, as long as you don't lose your Home Command Station you can still win the game.