Just finished first play through

I was really hoping for more in terms of difficulty I feel like other than ib-01 there weren’t many noteworthy bosses I’m not sure if my weapons were just that good but for a fromsoft game this was particularly very easy and underwhelming in difficulty, that being said I do absoultly love the customizability in terms of both coloring your mech and making builds and the arena while not hard was one of my favorite aspects each time I completed a chapter. My first playthrough I did the fires of ravens ending how do I unlock the next 2?