Welcome to DQ Forums @Vernand !
at this point, you need to decide what kind of build you want.
Farm - Gold, Crystal, Mythstones, Items, Dust.
Floor Climbing - Survival & Damage.
Battle Arena - Fight against other players Characters.
Ascending - Gaining Lots of Experience to get all Six Perks.
Eternal Items are powerful, but only 8(?) Crystals can be used on them.
if you’re lucky or observant, you can get far with random Legends & Eternal Items. eventually you’ll reach a point where you want a build with all of the affixes synergizing together. some players have made late game builds with Eternal Items, you just have to find the right one for your build.
one thing I tell new players, if you don’t understand how all the affixes work, just make an all Epic Affix build. it’s a great way to learn the basics and how the different affixes work together, and is relatively cheap, only needing common crystals. Items with Sockets will allow you to use Mythstones and see how they can fit into a build.