Piracy? Heck ya!
This whole it's undoable there's scads of patrol craft in the 100d, bah! There's worlds where the habitable zone of the star is within the star's 100d limit, there's habitable worlds as a moon of a Gas Giant, also within the 100d limit of the gas giant (we're on the third moon, just be sure n land in the radar shadow). Probably half the free traders are behind on their payments (skipjacks), smuggling, or out and out stolen (hijacked) craft! Over half the guys who come up and charge you for berthing fees and fuel aren't legit, or they're subcontracted out by a corporation, or bribeable. Forgery of the paperwork is quite possible as well! If you want protection, you have paid the 10kCr (or more!) for the one-use jump tape to the world, we patrol that spot regularly. If you're too cheap to buy a proper jump tape to get here, the quality of service you receive will be proportionate <s>. Probably half the XBoat traffic is lists of ships overdue on payments, tracking lost cargo, etc. whole corporations do nothing but transfer ships multiple times to gum it all up anyhow!
The belts of a system can get rough. real rough. Ships full of radioactives, gems, gold (!!!). Claimjumping, out and out (!) piracy. A lot of pirates don't even have jump ships, but want one pretty badly!
Half the piracy is a con anyhow: you jump in system, get yer traffic pattern (or whatever), in a "civilized" system likely get boarded by a boat of some kind to check your paperwork, look things over. You're met by a boat, a pirate boat! they are here to pick up one of the cargoes you have, cause there's now a rush order on the delivery. They're even offering half the 100Cr per ton rush fee. Paperwork's all in order even (maybe it is!). Or maybe when the hatch opened a platoon of vargrs charged in, whatever lol. Or a boat comes up to sell you fuel, only later do you realize you paid too much, or it wasn't refined etc.
IMTU by all means. It's canonical we have pirates in the ship encounter tables, we have a pirate career, the Aslan book has great stuff, privateers in *addition* to pirates, We have a Patrol Cruiser specifically designed to patrol the lanes, mention of running in convoys for safety for that matter (we have a mothly convoy. Payments you say? Yes we charge for the escort. oh *your* payment? well if you have a schedule to meet, you're on your own then!). Due to the nature of subsectors etc, there's lots of worlds, only a relative few are mid to hi pop starship capable with A/B ports. then there's the major XBoat trade routes. Any of those few are probably pretty safe, but even then there would be possible piracy through sheer amount of traffic/blending in. the rest of the worlds, probably a fair bit frontierish IMTU. Maybe semi heavily trafficked if they're on a main, worse if off it.
The whole SDB, well they are SDBs, they are not patrol craft. Their job is to remain hidden for response to a system attack. It's not their job to reveal their location, half of the pirate attacks would be like zhodani attempts to ID system defenses then anyhow. A highport is safe because it is armed! has internal security forces for order and response to boarding attempts.
Another version is even if a pirate ship say is boarded by the Imperial patrol craft, all they care about is do you have nukes? no? ok. got paperwork on the vessel, the crew, the cargo? great! Pay your 100Cr berthing fee when you dock, your berth orbit is X. Have a nice day!
Most of the canonical ships stay in orbit anyhow, transfer passengers and cargo through grav vehicles, launches, boats, or shuttles. If there's a highport, I'm pretty sure it'd be used. In beltstrike great example, there's a company charges for "port protection". If you don't pay, well, they only really make sure if any vandalism occurs, it's not on the ships they're paid to protect (maybe they even, well...). You buy a cargo, it's delivered. or did you have to hire a transport company to load it for you, or get it to your ship from the warehouse? (Stevedores! Mafia!! Think um, Hoffa) Maybe someone intercepted the cargo. etc etc etc lots (and LOTS) of possibilities. It would take a LOT of piracy to have a measurable effect on interstellar trade anyhow, heck misjumps alone allow for a lot of room for piracy.
My .02Cr all in good fun, and in the full spirit of IMTU people don't have to have it. Sure a lotta resources, skills, ships, careers, fluff you name it for it though!