<<And do pilots give sometimes (off-frequency) friendly advice on how to improve the ATCO’s RT performance.>>
Oh dear.... that brightened up my retirement no end, especially as I'm suffering from then dreaded 'flu!
When I was at LATCC the local examiners would be given copies of tapes of their "clients" for assessment and later discusson. Durinjg emergency training we would listen to tapes and learn from others' techniques. At larger, busy units standard R/T is essential but you only have to listen to smaller units to hear "lapses". One or two AFISOs I've heard think they're Space Cadets and gabble on the R/T, clipping phrases in the misguided belief that they sound smart. They don't; they sound like prats and I pity the poor PPLs who cop their nonsense.
Professional conduct is important, plus remembering to speak slower when things get hot.