Lvl 18 Rogue, what should I strive for?

[Shodo]

Depending on the damage of the other dagger, you may want to consider trying out weapon swapping for back stabs. Seb dirk has a great ratio, but things like frozen shard will backstab a lot harder since they have a higher damage. An easy thing you can do to get the better BS damage while keeping the seb dirk in your main hand is put both weapon slots on your hotbar, then pick up the higher damage weapon and put it in primary just before hitting BS, then switch back. With practice it should only take a second to do that.

As for other stuff:

Keep an eye out for dwarven work boots, bloodstained mantle and jagged band if you level in unrest.
Do the same for crested helm and hooded black cloak in mistmoore.
There are plenty of other drops to get too, and leveling in dungeons that both give good xp *and* drop upgrades for you is always an easy way to get geared. But don't pass up a juicy xp group just because you want a minor upgrade at a different camp - xp is a bigger upgrade than a few strength.

However, if you do get bored of the xp grind, you should be perfectly capable of grinding some cash with those weapons. At your current level, wisps in NK would probably be the most lucrative (can turn in greater light stones for a book that vendors for 10 plat and some xp). Really you're just looking for any camp that you can slice through so fast that the mobs won't be able to do much damage to you, since you can't heal easily.