Your gear looks good so far, I would suggest getting Excellion Wing, Excellion Suit, and 2 Expert Ring's to start. These items give you After Cast Delay, which reduces the time in between your casting of skills, essentially letting you Rolling Cutter more times every second. The other item I suggest is getting an Incubus (SP leech) or Succubus (HP leech) pet, these give either HP or SP leech when they are loyal.
I've got a guide here with all the current gears for the various GX builds, https://www.novaragnarok.com/wiki/Illegal's_PvM_Guillotine_Cross_Guide
We also have a discord server for NovaRO, and a separate channel for each of the class branches, this is the #thief discord if you are interested. https://discord.gg/WVVTrbH
We are quite active there, there's even a #support channel as well for nonclass related questions too.
Hope to see you around the server!