when I first started playing DQ, I tried out M3 early to see how far I could go. starting with my level 1 character. got up to around floor 30-40 I think, then I couldn't kill the bosses. so I stayed at very easy to normal for a long time, because M3 scared me. when I started Ascending for Perks, I started on the lower difficulties, and as I crafted my gears to be better, I eventually ended up on M3, have all the Perks, and am around floor 740.
use a difficulty level that you feel comfortable with as you learn to make better equipment. as you make the better gears, try them out on the higher difficulties. if they do well, then cool, if not, back to the drawing board and the lower difficulty level. that is what I did. sort of.