I've always imagined that SDBs operate much like a present day Coast Guard. Yes, they have significant military capabilities, and will be mobilized in the event of war (or serious threat of war). But during peacetime, they do other jobs like search and rescue, customs inspections, official transport of people and cargo, and a variety of other roles. Anti-piracy would be another role for SDBs, if a particular system is vulnerable to that in your TU. So most SDBs would be built as multipurpose ships, all with adequate weaponry along with good endurance, stealth, sensors, and independent-operations capability. But the details would vary a lot from there.