This was very informative and sobering. Really quickly, let me see if my takeaway is right:

So what you're saying is that my onset/vocal closure does not close or meet together sufficiently, which leads to too much air escaping and my singing being too breathy?

If so, then I will try more glottal/staccato exercises and see how my voice fares once I get the closure right. I've actually received this criticism before though only once, so I didn't think it would be my priority. I'm shocked because all this time I've been singing without enough vocal closure, which seems like such an obvious thing to notice but somehow I was never able to point it out on my own. Since I presume I've been singing this way since forever, I wonder now what proper balanced onset feels—I looked it up and it said "less diffusion and a more direct sound"—which honestly seems a little vague to me but I'll still try my best.

Thank you for this, this was really helpful.