Yes, killing off the crew of a ship should have triggered a bigger response. However, for the other points:
1. The high end prisoner was only high-end for Ahsoka. For the New Republic, she was just one of many warlords on outlying planets. Much like the U.S. doesn't militarily involve itself in heinous autocrats of various distant countries (unless it has direct negative consequences for itself).
2. They believed Thrawn was dead.
3. Everyone seems to know and accept that past imperial workers are present in the new Republic. As well as various criminal organizations.
4. The New Republic has endured years of war and genocides. They are extremely war-shy. They are also broke. They couldn't muster the number of agents or supplies to fight another war or police the whole galaxy. The previous republic had the resources, but Palpatine's plan to create non-stop warfare bankrupted everyone... except the Empire which relied on slave labor (cf. Andor), intimidation, and criminal organizations.
5. The theft of giant hyperdrives is meaningless to the New Republic with regard to what the thieves really wanted from it. They think it's just to sell for parts. And they don't believe Ahsoka because they believe Thrawn is dead.
6. Since old imperial agents are still around at all levels of government, some of the New Republic senators could be compromised, and they're blocking any investigation into a new order.
