Atarlost I'm thinking to do some changes to the Mass Drive to adjust the amount of bullets needed per second and also to make it shorter but keeping the same price.
I did some tests increasing the accelerators buff from 15% to 30% and the price from 2000 to 4000 and also reducing the firing speed from 3 to 4 seconds.
Here are the results at a similar cost (the amount of accelerators in brackets are with the changes I made):
Mass Drive +8(+4) accelerators(+120%) -> 22000cost with crew, 2200 dps, 220 range, 11 penetration
Railgun +0 accelerators(+0%) ---------------> 22000cost with crew, 2500 dps, 200 range, 15 penetration
Mass Drive +12(+6) accelerators(+180%) -> 30000cost with crew, 2800 dps, 280 range, 14 penetration
Railgun +1 accelerators(+20%) ---------------> 29500cost with crew, 3000 dps, 240 range, 18 penetration
Mass Drive +16(+8) accelerators(+240%) -> 38000cost with crew, 3400 dps, 340 range, 17 penetration
Railgun +2 accelerators(+34%) ---------------> 37000cost with crew, 3350 dps, 268 range, 20 penetration
At this amount of accelerators the Mass Drive can shot 8 times before starting to lose energy every now and then.
Mass Drive +20(+10) accelerators(+300%) -> 46000cost with crew, 4000 dps, 400 range, 20 penetration
Railgun +3 accelerators(+46%) -----------------> 44500cost with crew, 3650 dps, 292 range, 21.9 penetration
At this amount of accelerators the Mass Drive can shot 6 times before starting to lose energy often.
Mass Drive +24(+12) accelerators(+360%) -> 54000cost with crew, 4600 dps, 460 range, 23 penetration
Railgun +4 accelerators(+57%) -----------------> 52000cost with crew, 3925 dps, 314 range, 23.55 penetration
At this amount of accelerators the Mass Drive can shot 5 times before starting to lose energy too often.
At this point is better to start using Railguns due to the long time to recharge unless used for its range.
The Mass Drive is always longer and has more range, the railgun is always shorter but wider and has better penetration damage.
My idea of corvettes is on the size of 20 to 60 tiles long and Mass Drives of 12 to 28 tiles long.
I dont want to do diminishing returns neither capping the range at 400m because it will be too similar to railguns.
I hope with this changes the Mass Drive will feel stronger.
The energy turrets and the beam array they where never intended to be a main defense agains missiles, they are more like a support for the other main weapons (pd and flak).
The problem i have with increaseing the arc of the energy turrets its mainly the sprite:
As you can see if i increase the arc, the turret will go inside the armor and the crew member will be exposed to space.
I have increased the beam array arc from 180 to 190 and the damage to missiles so it can destroy at least a nuke every time it shot, still i has a very poor rotation speed so its very inefective agains anything else.
I think that the best thing for what you want will be to create an entirely new weapon or change the vanilla pd arc.