If you actually want to perform serious bombardment of planets you should consider using something like this for that purpose. You could add another engine and additional armour and shields if you want to use it during the assaulting phase to get rid of enemy STO as well. But it is mainly built for supporting ground troops for as cheap a price as you can get it, this cruiser should be able to fire it's weapons for a very long time.
Devastator class Bombardment Cruiser 20,829 tons 891 Crew 1,836.3 BP TCS 417 TH 1,250 EM 0
3000 km/s Armour 4-67 Shields 0-0 HTK 185 Sensors 8/8/0/0 DCR 10 PPV 230
Maint Life 2.90 Years MSP 1,351 AFR 347% IFR 4.8% 1YR 237 5YR 3,556 Max Repair 312.50 MSP
Captain Control Rating 4 BRG AUX ENG CIC
Intended Deployment Time: 12 months Morale Check Required
Ion Drive EP625.00 (2) Power 1250.0 Fuel Use 52.41% Signature 625.00 Explosion 12%
Fuel Capacity 640,000 Litres Range 10.6 billion km (40 days at full power)
25.0cm C0.05 Infrared Laser (40) Range 80,000km TS: 5,000 km/s Power 16-0.05 RM 10,000 km ROF 1600
20cm Railgun V10/C1 (10x4) Range 40,000km TS: 5,000 km/s Power 12-1 RM 10,000 km ROF 60
Beam Fire Control R80-TS5000 (2) Max Range: 80,000 km TS: 5,000 km/s 88 75 62 50 38 25 12 0 0 0
Stellarator Fusion Reactor R6 (2) Total Power Output 12.8 Exp 5%
Active Search Sensor AS6-R1 (1) GPS 16 Range 6.4m km MCR 574.5k km Resolution 1
EM Sensor EM1.0-8.0 (1) Sensitivity 8 Detect Sig Strength 1000: 22.4m km
Thermal Sensor TH1.0-8.0 (1) Sensitivity 8 Detect Sig Strength 1000: 22.4m km
You also could add/replace some smaller rail-guns for engaging regular infantry, the 20cm rail-gun will have a damage of 20/40 a 10cm railgun will have damage profile of 10/20... this is AP/Damage. The lasers are more against heavier vehicles with damage profile of about 31/63.
In general I prefer my orbital bombardment cruisers to deal with static and vehicles while my troops can deal with enemy infantry more economically.
The 25cm laser only cost 2 MSP when it fails and the 20cm railgun cost 3.5 MSP when it fails, so you can shoot a long time with these. Recharge times is of no concern for bombardment purposes.