Mind Spike is... a weird spell. I would fully recommend using it after you get your DoTs up, if only it didn't remove the DoTs.
Sometimes, it seems like there's a "sweet spot" moment where all your dots are running out anyways, and you have two options. You can either let them expire and use a Mind Spike or two before refreshing DoTs to make the next MB a guaranteed crit, or you can refresh the DoTs and continue as usual with Mind Flay/Mind Blast.
They've done work on this subject, and it turns out that refreshing the DoTs and continuing as normal will actually do more damage every time.
I know it seems tempting to use MS, but I really caution against it. The only time I've really found that Mind Spike is preferable is when the mob is going to die before your DoTs can go their full duration. You have to think ahead: if you will have to refresh the DoTs before the creature dies, use DoTs. If it will die before the DoTs do, use Mind Spike. Short-lived trash is an excellent example of when to use it. Also, I used it almost exclusively using it while solo leveling, just because I found it to be more mana efficient.
My memory is vague about what I was doing at those levels, but to me it sounds like you're doing just fine. You didn't mention it, but I'm sure you're doing it, but use SW:D while the mob is under 25%. I found that Masochism + the Glyph of Spirit Tap + the Glyph of Shadow Word: Death = nearly infinite mana. The glyph of SW:D is also a huge dps boost. Think of it as our Execute.