mobile structures that are not starbases ain't possible.  Create an ability that deploys specialized beam turrets that do module damage.  DUH!  Ability that deploys them can release them but ai is dumb and you'd probably like to dump them right up close to the starbase or at least in weapon range.  speed is -1, but if you wanted them mobile to get at the starbase because, like the TEC capital that dumps missile batteries in inappropriate places, you can give them mobility and write it up as the Advent's TK abilities at work and be able to send them into a gravity well with an enemy starbase.  Would also be best to make the damn planetary turrets do Capital damage.  Antimedium class weapons do half damage against module armor.  Capitalship damage does 75% of its listed damage vs module.  A boost to damage beyond 20 DPS (180d/9s) might make them a bit more fear inspiring, but as above and post below... there are ways.

NM what 7DS has done... just think of what's below. 

**FrigatePsiOMGWTFSTARBASEBBQER.entity** This one is stationary and only targets structures.  perfect antistarbase turret, but it'll only attack when starbase is being deployed so rush your ships at the starbase constructor and wait for the bugger to start building it where your stationary turrets can't hit it, then deploy the turrets around it and blast the bastard!
TXT
entityType "Frigate"
defaultAutoAttackRange "None"
defaultAutoAttackOn TRUE
prefersToFocusFire TRUE
usesFighterAttack FALSE
autoJoinFleetDefault FALSE
canBomb FALSE
typeCount 1
frigateRoleType "Lrm"
statCountType "FrigateSpawnedByAbility"
mainViewIcon "MAINVIEWICON_PLANETMODULE_PSIORBITALBEAMDEFENSE"
picture "PLANETMODULEPICTURE_PSIORBITALBEAMDEFENSE"
NameStringID "IDS_PLANETMODULE_PSIORBITALBEAMDEFENSE_NAME"
DescriptionStringID "IDS_PLANETMODULE_PSIORBITALBEAMDEFENSE_DESCRIPTION"
counterDescriptionStringID "IDS_FRIGATE_TURRETCOMBAT_COUNTERDESC"
basePrice
    credits 0.000000
    metal 0.000000
    crystal 0.000000
slotCount 0.000000
BuildTime 1.000000
hasLevels FALSE
ExperiencePointsForDestroying 16.000000
MaxHullPoints 3000.000000
MaxShieldPoints 424.000000
HullPointRestoreRate 1.000000
ShieldPointRestoreRate 1.000000
BaseArmorPoints 5.000000
maxMitigation 0.700000
Prerequisites
    NumResearchPrerequisites 0
numRandomDebrisLarge 0
numRandomDebrisSmall 7
numSpecificDebris 0
armorType "Module"
hudIcon "HUDICON_PLANETMODULE_PSIORBITALBEAMDEFENSE"
smallHudIcon "HUDICONSMALL_PLANETMODULE_PSIORBITALBEAMDEFENSE"
infoCardIcon "INFOCARDICON_PLANETMODULE_PSIORBITALBEAMDEFENSE"
minZoomDistanceMult 1.500000
NumWeapons 1
Weapon
    WeaponType "Beam"
    damageEnums
        AttackType "ANTIMODULE"
        DamageAffectType "AFFECTS_SHIELDS_AND_HULL"
        DamageApplyType "OVERTIME"
        DamageType "ENERGY"
        WeaponClassType "BEAM"
    DamagePerBank:FRONT 180.000000
    DamagePerBank:BACK 0.000000
    DamagePerBank:LEFT 0.000000
    DamagePerBank:RIGHT 0.000000
    Range 7500.000000
    PreBuffCooldownTime 9.000000
    CanFireAtFighter FALSE
    SynchronizedTargeting TRUE
    PointStaggerDelay 0.750000
    TravelSpeed 0.000000
    Duration 4.000000
    fireConstraintType "CanAlwaysFire"
    WeaponEffects
        weaponType "Beam"
        burstCount 1
        burstDelay 0.000000
        muzzleEffectName "Weapon_PsiCapitalBeamHeavy_Muzzle"
        muzzleSoundMinRespawnTime 0.100000
        muzzleSounds
            soundCount 1
            sound ""
        hitEffectName "Weapon_PsiCapitalBeamHeavy_Hit"
        hitHullEffectSounds
            soundCount 1
            sound ""
        hitShieldsEffectSounds
            soundCount 1
            sound ""
        beamEffectSounds
            soundCount 1
            sound "WEAPON_PSICAPITALBEAMHEAVY_TRAVEL"
        beamGlowTextureName "PsiCapitalBeam_Glow"
        beamCoreTextureName "PsiCapitalBeam_Core"
        beamWidth 30.000000
        beamGlowColor ffffffff
        beamCoreColor ffffffff
        beamTilingRate 3.000000
m_weaponIndexForRange 0
firingAlignmentType "Default"
TargetCountPerBank:FRONT 1
TargetCountPerBank:BACK 0
TargetCountPerBank:LEFT 0
TargetCountPerBank:RIGHT 0
canOnlyTargetStructures TRUE
hasWeaponLevels FALSE
mass 2000.000000
ShieldMeshName ""Shield_ModulePsiBeamDefense"""
renderShield TRUE
maxAccelerationLinear 0.000000
maxAccelerationStrafe 0.000000
maxDecelerationLinear 500.000000
maxAccelerationAngular 3.000000
maxDecelerationAngular 9.999996
maxSpeedLinear -1.000000
maxRollRate 2.000000
maxRollAngle 35.000000
squadTypeEntityDef:0 ""
squadAntiMatterCost:0 0.000000
squadTypeEntityDef:1 ""
squadAntiMatterCost:1 0.000000
squadTypeEntityDef:2 ""
squadAntiMatterCost:2 0.000000
squadTypeEntityDef:3 ""
squadAntiMatterCost:3 0.000000
maxNumCommandPoints 0
NumSoundsFor:ONATTACKORDERISSUED 2
SoundID "CAPITALSHIP_TECHCARRIER_ONATTACKORDERISSUED_0"
SoundID "CAPITALSHIP_TECHCARRIER_ONATTACKORDERISSUED_1"
NumSoundsFor:ONCREATION 0
NumSoundsFor:ONGENERALORDERISSUED 0
NumSoundsFor:ONSELECTED 1
SoundID "CAPITALSHIP_TECHCARRIER_ONSELECTED_0"
NumSoundsFor:ONSTARTPHASEJUMP 0
MeshNameInfoCount 1
MeshNameInfo
    meshName "PlanetModule_PsiOrbitalBeamDefense"
    criteriaType "None"
meshNameIncreasedEffectName ""
meshNameDecreasedEffectName ""
ExhaustParticleSystemName ""
ExplosionName "Frigate0"
HyperspaceChargingSoundID "HYPERSPACE_CHARGEUP"
HyperspaceTravelSoundID "HYPERSPACE_TRAVEL"
EngineSoundID ""
ability:0 ""
ability:1 ""
ability:2 ""
ability:3 ""
ability:4 ""
MaxAntiMatter 0.000000
AntiMatterRestoreRate 0.000000
cargoType "Invalid"
maxCargoCapacity 0.000000
formationRank 0
minShadow 0.000000
maxShadow 0.500000