When I talk piracy, I talk "deep space/high seas" piracy.
The issues are not tied solely to the Imperium.
First, the window for piracy is the jump window. Those arriving at 100D and traveling inward, and those traveling outward.
This is a very small window, all things considered. A matter of hours. Obviously there are exceptions based on jump shadows and such, but much of the travel is within the 100D limit, which is not very big.
Space combat is very expensive. Space ship parts are not cheap, and expensive to repair and replace. This works against pirates. As they say "The authorities can miss over and over, the pirate can only miss once."
If piracy is at all a reasonable risk, then the polities involved are either obligated to patrol the 100D space are, or the merchants end up arming themselves. They arm themselves not so as they can "take on" pirate ships, but only to reinforce that if the pirate wants the prize, it's going to be very expensive.
Piracy affects trade, systems like trade. Patrols can be expensive, but likely less expensive than losing trade. If trade is expensive, if merchants have to price in the risk of piracy, then prices go up to the point of even having trading stop.
Patrols do not have to patrol the vastness of space, only the 100D jump lane. If merchants want safe passage, they'll travel where the patrols tell them to travel. Effectively convoying from orbit to the 100D mark.
Obviously, a coordinated pirate attack can overwhelm local patrols, but that doesn't mean they can instantly make some system a safe haven for piracy. Word will get out, ships will stop coming. It's simply not worth it. The pirates may well be able to take over a system as a refuge to where the local systems can not rally enough forces to root them out, but that system would just go under embargo and merchants would stop coming.
If pirates decide to start making coordinated attacks on systems, looking for low hanging fruit, once they've dealt with the local patrol, of course, then the game changes and the other systems will have to rally and organize to thwart them.
That could be an interesting experience, but whether the pirates can hold on in the end, I dunno. They may make an interesting footnote in history, but I think long term the governments will be able to reduce them effectively.
It would be a very interesting small ship campaign of pirates vs organized, armed merchants/privateers.
In the end, as much as the merchants want to trade, it's not worth getting killed over.