In my experience playing and modding this game to all hell i have learned many things that i wish i had been told.
Thing one: Infantry are usually useless. Besides providing a literal meat shield for your larger vehicles and capturing things, all infantry are generally a support unit, rather than mainline combat units. exceptions are the two clone wars series mods.

Thing two: point defence ships are vital. having a fleet of smaller ships who can zoom around popping proton torpedoes (and smashing the dudes who shot them) is an awesome counter to the outright ridiculous number of missiles the AI will throw at you. learn what ships can stop or jam missiles, and always keep a few on you.

Thing three: ship captains are idiots. while some mods have enhanced the pathing of the base game, the outright derpiness that comes out of the galactic military colleges should be watched with careful attention. when you give move orders, pause the game, press "w" and carefully plot out a path for your advancing vessels. the ai doesn't do this, which is why you slaughter them. triple important for the bigger ships. if one ship starts trying to do a spin, click it and hit "s". this will stop it in its tracks and allow you to replot its course. Just as well ships will try to move to a unit that they are ordered to attack. DO NOT LET THEM. they do not know their best firing arc, and will pass the thing they are shooting at if left alone.

Thing four: fighter pilots are also idiots, but microing fighters makes them extremely effective. keeping them away from concussion missiles, leading them into bomber wings, and ensuring the safety of your precious bombers is vital for your success. pausing and ordering may not be effective due to how fluid fighter combat is.

Thing five: learn yer OP units. Minor oversights often result in some units being super strong or super weak. this is par the course for the modding community. AT-RTs are usually op, AT-ATs are usually flop. find this out before mass producing an army.

Thing six: master your industries. using the four yellow buttons next to the galactic map allows you to see a planet's building and space level limit, credit income, and heroes on the planet. worlds with big base incomes will have better bonuses to mines built on the planet, while mines built on smaller income worlds will not pay for themselves in 20 turns. worlds with poor income should instead be relegated to unit production. you can send units built from these world to defend mining planets if the need arises. the ai loves snapping up mining worlds.

If i've forgotten anything you have my apologies.