But you can't do that. You can't handwave the rules away and "come up with ideas", since the "rules" provide the foundation for your thesis.
My system defense fleet is a buoy that lets me know that ships are encroaching so that I can hyperspace my fleet of Tachyon gunned dreadnoughts via the worm hole projector from where the crew likes to shore leave around this garden planet orbiting around a quiet star in the Andromeda galaxy.
When they arrive, they use their quantum ship detectors and use the tachyon guns to englobe the enemy in particle fusion nets -- where they just implode and destroy the enemy ships. Then the fleet returns by dinner, because Layla is dancing tonight -- and they don't want to miss that. They love Layla.
Actually everything except the stellar X-ray laser can be done in TNE, T4 or T5, and I suspect the Stellar X-RAY can be done in T5. It is a serious call to think up stuff that can be done to defend a system in tandem with a fleet.
C fractional missile...
start with one of these:
Hull
TL 15 Enhanced bonded superdense 1m3 armor 11 stressed to 19 g's toughness 47.1 density 15
Sphere 1.24m dia 4.84 m2 SF = .0000044
Armor 11/47.1 (minimum.25cm) .25cm * 4.84/1000 = .0121m3
SF 19*.0000044= .0000836
Total armor volume .0122 mass .183mt
Power (nominal1 MW) TL 15 fusion .168m3 .336 mt 1.008MW -.001m2
pp fuel .008m3 .00057mt 292 days endurance
Heplar .1m3 .1mt -1MW 200KN
Payload .1m3 .16mt SR 10kt nuc det laser DE200 DV50 PV286 (3 major hits per BL)
Computer CP1.67 .005 m3 .01mt -.001MW -.0043MW
Battery .05m3 .05mt [.3MW/h]
PEMS Sensor sens 12.5 .05m3 .05mt -.005MW-.1m2
LIDAR Tracker sens 13.5 .1m3 .2mt -.05MW -.05m2
Maneuver fuel .55m3 .039mt (2.2 hours @17 g's)
1000Au Laser comm .015m3 .03mt -0003MW -1.0 m2
Total Mass 1.14mt + maneuver fuel/2 =1.16mt
1.07/200=186m/s or 19 g's at Earth 9.8m/s G's (18.8g at standard)
add a 2 Dt fuel tank that grapples the missile to give you some 28.5 KL of fuel used at .25kl/hr and you have something that averages about 10 g's for 100 hours+ (3600*100*100/1000) = 36000 km/sec or 12% of C. 1/8 C I think qualifies as C fractional as it takes 1.11 min to move from 8 light seconds to detonation range. (the last two hours you detach from the fuel tank and vector in with 19 g's of agility) 8.3 hours at that speed to reach a light hour, with 112 hours of thrust this missile could indeed reach nearly anywhere in the solar system in about 4 days and the 1000 AU laser comm may need more range.