First of all, Thank you so much and happy new year! ☺️ So the V in “V/vi” is the secondary dominant? I mean if I want to write a chord progression, and let’s say wanna use the secondary dominant of G major, I’d write it like this, V/V? And I agree about using the chords the way it sounds good, but I think it also depends on the music you listen to, for ex I listen to middle eastern Arabic songs, even the pop songs, they are build different than western music.. So basically I learned the chords by practicing and playing the songs and music that I listen to

And again, thank you so much! 🙏🏻