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.
Idk man, i reckon you will be better off with missile defense plataforms (stations) and make it big and build with factories on your homeworld. If you are going with 500 tons missile pods, a 60k Missile Defense Platform will be a better choice. Also, if you have to go with 500ton fighers to defend colonies, you are better off with going full FAC, go slightly behind 1k ton and make it more robust. Maybe put some railguns, energy weapons or something like that. Also, if you have to defend a colony, might as well move factories there and make 7k defense platforms, one at the time. Just leave them hanging on top of the colony, make the deployment time low and make like 10 of those. If you really want fighters, then make box launchers with some torpedoes, something around 4-6 damage missile, made for being thrown at close quarters and with enough agility to evade PD defenses. You then cut down their tonnage by as much as you can and make another defense platform to be used as a hangar (Even tough you dont need that because you have a colony, you can use it to RP if you want).
Couple that with heavy static AA guns, STOs and you have a good mix there. You will at least buy some time till help arrives, if you can.
However enemies will be over 50000k tons worth of troops. So your best changes are to thin them down and to do that you will need an effective defense platform. Those will be able to make a scratch.
(Assuming you will have like 60 fighters at least to back all that up. I would put FACs as well. About a dozen.)