You need to differentiate between glans size and firmness - what makes it bigger won’t necessarily make it firmer. Back when I was working on glans size I did squeezes, and jelqs where I held the blood in the glans for a few seconds at the end of each stroke. Pumping can help too, of course.
The really hard and full erections might need other approaches - lots of kegels and edging, regular gentle PE with rest days. And exercise, sleep, and good diet.