One thing from Traveller you might think about is the system defense boats. They're just ships without jump drives used for in-system defense, patrols, search & rescue, etc. Removal of the jump drive frees up space for bigger engines and better armor/weapons. The shorter duration of the mission reduces living & stores space, and there's no expectation or the crew needing to do real repairs (worst case is pop the escape pods and wait for another boat). They are (by size) faster, tougher, more agile, better armed, and significantly cheaper than a jump capable defense fleet.