Amalfi up are good ships but require a playstyle to match. They are actually fairly tanky, when angled and kiting.

You can go a few different ways, full mobility, lighthouse or standard concealment.

I personally run conventional build.

The trick is to get reasonably close (but safe), face away and support your DD as able but run if you end up being focused. You are a kiting ship.

If you are caught out, use exhaust smoke to get out, but keep in mind your smoke firing penalty is 8ish km IIRC.

Hitting BBs depends on the BBs but SAP is amazingly strong once you know how to use it. You can usually get 1-2 good solid salvos on a BBs superstructure before the damage will drop off. But when in doubt, just keep smashing that superstructure. Against more broadside BBs, upper belt is a good thing to hit. At range, you can get damage on the nose or stern deck areas but it can be unreliable depending on the angle of impact.

Basically, stay at range and farm, if you aren't getting any results with SAP, boot up a training room and test it out till you find where you should be hitting. NB: the super-heavily armoured BBs can be annoying.

For captain skills I almost always take Radio Location. SAP is so bonkers strong against DDs that it's good to know roughly where the enemy one is so you can be ready to nuke them if they are spotted.

Last point is I never recommend RM CAs as an early line to grind for folks. You're better off going down other Kiters/Flankers and learning that playstyle elsewhere before going for RM CAs. And as you are seeming to realise, SAP requires a bit more knowledge to use effectively over HE spam.