i've been a cutter since age 14. i wouldn't say it's a way to build up to CTB, or makes me want to CTB more. its really different for everyone though, personally though my self-injury is completely separate from my suicidality.
i really don't recommend turning to self-injury. it can turn into something resembling a full blown addiction if you aren't careful, and it can make your interpersonal relationships strained. i'm not sure of the exact science, but it does give you a little bit of a ""high"". it never lasts long though, and soon youre just left at square one. the stigma around self-injury is also not a great one! people take one look at my scars and think im emo or think im trying to manipulate other people/get attention, when in reality i do it because it feels good to me and calms me down. it really sucks and its awkward! i really really dont recommend it.
its a really tough habit to kick and a really tough habit to hide to boot! i know you intend of CTBing sometime soon, so you may think you don't have much to lose, but in the event that things get pushed off, you could be left with a new longterm problem on top of whatever youre already dealing with