Some info on prizes (ships to go after for loot)
Easy
Independant traders, especially in Shaydensea, over the northlands and Curselands - in the Stormcircle is more likely to be profitable, but riskier due to Cartel patrols.
Troll trade ships can be hard to take individually, but you're less likely to suffer reprisal from Trollhaven.
Challenging
Spire Cartel Trade Ships are well armed but can be profitable, may draw the ire of the cartel.
Engineer's Guild Ships can be hugely profitable but are often heavily armed with unique and unpredictable weapons.
Other pirates because the sky ain't got no law and if you can't defend yourself you will not prosper.
Suicidal
Imperial Traders will impress Mandalthraxus but pits you against Imperus' merchant navy and there could be members of Their elite forces onboard.
Great House Traders very likely will have Scions onboard and can result in a very personal vendetta with the owning House.
Inquisition ships are a coinflip at high cost - they could have something obscenely valuable onboard, even just intel that can be sold to nation states, but Inqusitors and their retinues are absolutely no joke in a fight.
Edit: many typos I got two hours sleep and just finishing muscling through a workday