I'd like to give some tips to CA,CL players fresh from blitz.

Okey first off their are 3 things you can do in a CA,CL in GB

1. Anti-Aircraft Support (for this I'd recommend a +12 base reload sailor and for every nation keep it classed as a reload gunner or gunner and class them on time as they lvl, except for KM which can be classed as AA gunner (however, there are other methods where you can still reach reload cap and have an AA gunner from another nation although AAW is pretty useless against high lvl planes) and still retain it's reload ability)
2. Anti-Sub Warfare (for this I'd recommend a sonarman in the S slot and DO NOT OVERHEAT IF SUB IS NEAR sonar will NOT work if overheating and has a cooldown when coming out of overheating, use reload gunners if using Hedgehogs and if using Depth charges then make sure you have bulge/ good engineers to survive your own DC's and any gunner will do)
3. Anti Ship/escort duty (for this I'd only recommend higher end CA's for their range as CL's need to get too close to the enemy and will be suceptable to BB fire, use L type guns with Light HE rounds for maximum range)

AA-(ALWAYS USE AIMING FCS) For AA the best strategy I have found is too follow the BB line and kill enemy scout planes as they approach and to protect the BB's from Dive Bombers. Make sure to stay on your teams side of them, (just to the left of them if bravo,right if alpha). Keep your AA guns angle set at the "Golden Angle (See bottom for links)" for aircraft at max height. For aircraft at other heights it will just take practice to memorize the right angles.

ASW-(Aiming FCS is okay but I have found that auto is good due to the fast reaction times needed in ASW) Another thing on Sonarmen, you need to roll a sailor with high potential and class him as a support sailor. For this role you can choose to follow battleships or in the beginning you can watch the enemy side to see where the subs are heading and actively hunt them. IF YOU SEE A FRIENDLY SUB IN BATTLE WITH THE ENEMY SUB wait for them to finish their fight!!!! SS players HATE IT when they get killed by their own team's ASW!!! Just like AA any boat with large numbers of gun mounts is best for this. Though depending on whether you use HH's or DC's you will need to look for large amounts of R-slots or T-slots. There seems to be no best nation or ship for this.

AS/Escort Duty- (Aiming FCS is best) For this you will want to follow the same strategy as AA. Follow the Battleship line and make sure any CA's and under that are acting as scouts are killed quickly and DO NOT distract your side's battleships! Using DD's to scout and distract the enemies BB's is a good and well known strategy!

In my opinon pre-BB CA's make the best support ship in GB because in certain cases they can combine all 3 roles into one ship. Do not be fooled into thinking they are useless! There is a use for every ship class in the game!  

Edit: I forgot to mention how important using scout planes is as a CA/CL support ship. Make a sailor with high aircraft. KEEP IT CLASSED AS ROOKIE PILOT or you will be unable to use scout planes!

Golden Angle: Use the following Link to learn more about the Golden Angle and a list of Golden Angles for every nation's AA guns. https://www.nf-guides.com/aa.htm

P.S. The AA role is the best way to farm experts for your sailors, in my opinion, short of buying them from the store.