There's an option I haven't seen mentioned yet - what I call "missile pods." In effect, they are 500 ton fighters without an engine, filled with as many box launchers as possible and a single missile fire control.
Their role is very simple - to provide the defended colony with a standoff offensive capability to retaliate against hostile fleets that may attempt to bombard the planet into submission from outside STO range. While I always give my colonies decent CIWS STO weapon capacity to guard against exactly this, a sufficiently large fleet might manage to saturate those defenses and inflict grievous harm on the colony. As CIWS STOs are destroyed, effective hits from follow-on salvos will be bigger, and soon the colony defenses will be defeated.
Missile pods offer a very cheap and easy solution to this. Since they have box launchers, they can expend their ordinance in one massive salvo, drastically increasing their chances of saturating hostile defenses unless they have a massive advantage in numbers, tech or both. Full or even substantial destruction of the hostile fleet isn't needed - just enough damage inflicted to reduce their salvo weight below what the planet's CIWS can reasonably and reliably intercept every 5s tick.
I usually build these early on to quiet my restive home-world populace by providing PPV, even if the launchers aren't loaded. Later on, they can be loaded with older, obsolete missiles I no longer need for frontline service. When I'm setting up defenses on outlying new colonies, I'll typically dispatch a freighter to deliver minerals (if they're not being mined on-site) to build maint. facilities and for MSP production, a troopship to deliver STO weapons (both anti-ship and CIWS batteries) and one of my CVLs loaded with missile pods. If a CVL isn't available, I'll just build a station with sufficient commercial hangar space, load it with missile pods, then have a tug tow it into location. Then I can either tow it home to use for other such transport duties, or leave it on-location if I want my missile pods to be able to reload (in which case I'll provide enough commercial magazine space on the station to hold all intended reloads, obviating the need for planetside infrastructure if I wasn't planning to build it for other reasons.)
Included in this concept is a "sensor pod," another engineless fighter that only mounts an active sensor to spot for missile fire. I typically build two per missile pod defended colony, for redundancies sake.
With enough missile pods, you can get the throw-weight of an orbital defense station (or three) with only a little extra tonnage to maintain. A proper station using magazine-fed launchers will be more tonnage efficient, but you also have to tool a yard for it. I prefer to use my yards to build proper warships, which can also project power throughout the star system. I like building fast, beam-armed FACs for this purpose. Typically I end up liking those designs enough that my third or fourth CVL will end up loaded with FACs instead of fighters (esp. since I already have a 4-slipway, 1,000 ton naval yard tooled for them.)
Strictly speaking, missile pods can be used by carriers in this fashion as well, though I've never had to resort to such desperate measures. Carriers should never get that close to the enemy.