well if you already have the original sins just buy the expansion set, its cheaper and will save you $20

Entrenchment would be the next step up to try as you are given a lot of new defense toys (starbases, etc) but the AI isn't even close to being able to handle them and the pirates are still stunted

afterwards go for Diplomacy which has and upgraded AI (still not up to caliber but can give newbies a hard time) and a mean nasty set of pirates

Play with the pirates off in diplomacy, at least at first. Bombers are actually the best way to deal with starbases, even though the game would make you think anti-structure units were. Don't underestimate the pacts.

The diplomacy pirates were tuned up in strength to challenge the veterans, so they absolutely murder everyone else.  The veterans still dislike the pirates, despite being a serious threat now, because the feature remains predictable and distracts from the real battle - against a challenging and cunning opponent.  As a result, now no one likes them...