Jumping back to the math and point defense efficiency again, maybe throwing gun tonnage and range at area defense is actually not the optimal solution. The efficiency is very dependent on the relative speed of the vessel/missile - both for tracking speed and for engagement time.
If you know you are going to face a big burp of missiles, it might be most efficient to send your beams out to the missiles, rather than letting the missiles come to your beams.
If you could design a light craft that was basically all engine with one gauss, such that it can keep pace with a barrage and ride it in sitting at 10km astern, you could potentially blatt down the whole barrage over the course of a few million km following it in AD mode. Say 2000-4000 tons of max boost engine and one or two 10mm rails or reduced size gauss and a BFC. You would not even need a turret for such a high velocity craft.
Of course, I'm not sure if that is actually workable in practice, since missile boost can be higher than ship boost. It might require a significant tech advantage to pull off. Fortunately for my play through, I have the tech... bwahhaah.
You would still need sufficient detection range to see the incoming FAC wave and send your own spoiler ships out to meet them before they launch. You may also need lots of pickets to detect inbound missiles if the FAC launch is at too long a range. Long range missiles may be better for you tho if you can detect them, since more fuel = more weight = less missiles or slower missiles.
Edit: Oof that engine costs a lot to research xD
Oh god what have I done this thing feels wrong:
Alcibiades class Corvette (P) 3 884 tons 221 Crew 1 584.2 BP TCS 78 TH 2 880 EM 0
37081 km/s Armour 1-22 Shields 0-0 HTK 20 Sensors 0/0/0/0 DCR 1 PPV 9
Maint Life 0.38 Years MSP 254 AFR 121% IFR 1.7% 1YR 671 5YR 10 058 Max Repair 720.00 MSP
Adept Control Rating 1 BRG
Intended Deployment Time: 3 days Morale Check Required
Magneto-plasma Drive EP1440.00 (2) Power 2880.0 Fuel Use 450.00% Signature 1440.00 Explosion 30%
Fuel Capacity 100 000 Litres Range 1.03 billion km (7 hours at full power)
10cm Railgun V10/C3 (3x4) Range 10 000km TS: 37 081 km/s Power 3-3 RM 10 000 km ROF 5 1 0 0 0 0 0 0 0 0 0
Beam Fire Control R48-TS20000 (1) Max Range: 48 000 km TS: 20 000 km/s 43 32 20 9 0 0 0 0 0 0
Active Search Sensor AS4-R1 (1) GPS 4 Range 4m km MCR 360.5k km Resolution 1
This design is classed as a Military Vessel for maintenance purposes
To hit is vs 37000km/s missile
(yeah I forgot a power plant but you get the idea xD)
Of course this could be countered by not launching until close range since this thing has no armor and can't actually tangle with a beam FAC escort but at least you'd force some sort of beam fighter engagement in that case I suppose. Also, they could switch to a faster lower payload missile a lot easier than you could speed this monster up, but at least you'd sort of have accomplished your mission of lessening the volley size in that case.
Hmm yeah but it looks like at similar tech I can make a 40-50k missile pretty easily so... poo, lol. You can continue to strap more engines on this thing but it starts to get a bit absurd - 3 and 4 engines are basically the same speed (40kkm/s) because the engines are just pushing more engines at that point, haha. Removing fuel or guns will buy you a bit more but not too much.
I suppose if the missiles are just one or two kkm/s faster you could switch to lasers and still get a large number of shots off, but for a bigger speed differential it isn't really worthwhile. Also they could just, you know, shoot you in that case, but I guess that also would draw some of the volley off target after a fashion xD. Lasers would potentially let you snipe down the launching FACs tho, which is an interesting twist.