Gentlemen, ladies, thank you all kindly for your responses. Thank you for the motivation and also great advise to consider and take onboard! I will endeavor to read each one.
Cheers!
To summarise, regardless of auto-thrust mode, use of speed-brake will result in an effect if the aircraft is beyond its speed target. (In the scenario of descent)
However to be noted that in the case of SPEED mode if the auto-thrust can not maintain a target, due to excessive V/S then reaching VLS or overspeed could occur.