They have a population of tens of millions or hundreds of millions if you're in the Taurian Concordat or Magistracy. The Outworlds Alliance showed lower populations and some worlds of the Outworlds Waste could be uninhabited after 2850. Campaign Operations provides a rough estimate of how planetary populations tend to drop with distance from Terra. Distant worlds received fewer colonists later in history, giving less time for growth than a core world.
I've got a few points to address on this one. You bring up some good points and you are correct on every account of these being ways to end the pirates reign of terror who falls into one of these traps. These are the methods that are always going to be used, and always have been used against pirate attackers.
Pirates are constantly analyzing the risk versus reward matrix. High risk = high rewards but higher chance of getting killed. Hitting more organized worlds or well defended convoys, settlements etc. might equal larger loot but you are more likely to get your group killed. Pirates aren't going to be very successful doing this often. They might get famous hitting these bold targets, but a more successful pirate will hit low risk targets more frequently and come out better off. Your point about the deeper periphery worlds having lower populations just goes to bolster my initial point. The keys to piracy are:
Legal and jurisdictional opportunities
Conflict and disorder
Under-funded military / law enforcement /inadequate security
Permissive political environments / Cultural acceptability
Reward
Pretty much all periphery worlds have these factors going for (or rather against) them. When you factor in state sanctioned piracy and warring interstellar states it makes piracy even easier. Not only piracy as a direct action, but terrorism, mercenary work or the classic "non-state actor" role all offer pirates other mechanisms to gain money. With jump ships and interstellar travel opportunities, a pirate force can call a low population / (otherwise uninhabited) planet home base. From there pirate forces can gather intel via spies, interrogations, or other intelligence gathering. A well planned raid will occur quickly.
If pirates are going to linger on a high population planet then:
1. The pirates are going to get out of their 'Mechs at some point. Then they're vulnerable to snipers, prostitutes' knives**, and arrest.
2. The pirates give the locals plenty of time to set up IEDs and drive truck bombs to their barracks/DropShips. ("It's your fifth shipment of gold, promise!")
3. The pirate 'Mechs are going to need maintenance, per Campaign Operations' force operation rules.
1. Of course they are, but whoever the pirates are attacking are just as vulnerable to these things, potentially more so because the pirates are not operating under the guise of any sort of legality. So a clever pirate might send advance "Scouts" to a world and assassinate their military commanders / mechwarrios, sabotage equipment, destroy bridges, spaceport infrastructure etc. These are all ways of pulling off a more successful raid.
2. If pulled off correctly the locals aren't going to know the pirates are hitting until they do. Ideally pirates would jump into a (pirate point) and make planetside as fast as possible. Planetary defense systems are going to be minimal at best in the periphery in particular but even in inner sphere backwaters they will be limited. From this point out its a smash and grab raid, set a time limit and objective and start carrying it out. The longer it drags on the higher the changes of "getting caught" one way or the other. As a pirate leaving a dropship in an indefensible location is tantamount to stupidity. A dropship though, could take off after dropping the pirates off and land somewhere uninhabited, then return to pick up the pirates at a DZ. Putting the shoe on the other foot what do you think a periphery mining company would pay off to avoid pirates from destroying / seizing a mid-charge jumpship or a dropship for that matter? The pirates might not even have to attack. With some decent spy work from the pirate side pulling something like this off couldn't be too difficult.
3. This is probably a portion of the reasoning behind the raid to begin with. We aren't talking about a protracted campaign but something more like Henry Morgans raid on Portobello. He hit Portobello and then contacted the Spanish after looting the town. Morgan demanded ransom or he would burn all of Portobello. Spain paid off. I think a lot of periphery nations would act similarly when facing losing a major settlement
Again, successful pirates are either going to get enough loot to cash out and retire (which shouldn't be too hard with battlemechs) or they are going to end up going privateer / mercenary type route. Considering most periphery nations are pretty poor, getting a contract with the enemies of who you were raiding should not be difficult for you. But for every smart / lucky pirate that pulls these things off there will be 10 that fail. As your pointed out there are so many pitfalls / honey traps / general difficulties to avoid. But it is the
prospect that keeps them coming back. After all the root word pirate means to "try or to risk". Given high enough reward, there will always be many who will risk it.