I believe that would mean you have 21.3.0, yes.
Essentially yes, here's a short updating guide on it.
The TWiLight Menu++ version will be shown at the bottom-right corner of the top screen of the settings menu, with the nds-bootstrap version on the top left. Some specific games may have problems, but it's fairly unlikely.
If you do run into problems with TWL clock speed, you can let us know so that it can be added to the blacklist.
I dunno if sound frequency is known to be problematic, though.
Many thanks for your help and prompt response! I've easily updated the TWL Menu to the latest version and it's working good.
So that means that the console won't get any damage for being overclocked, is that correct?
It's been a long time that I've been hacking and updating most my consoles, but I still feel like a noob when it comes to things like overclocking and changing certain settings using CFW, hence my questions, to try to understand if I'm on the right track.
Cheers.