I'm level 18 Wood Elf Thief with maxed sneak skill and short blade, archery, unarmored between 50-60. I've been using a mod that prevented DB assassins to appear too early in the game but now I've reached the rank of Master Thief and he appeared in my home when I was resting. Now I'm in a dilemma: from a RP point of view my character would like to immediately find out who's attacking her but on the other hand I'm thinking if I'm powerful enough to start Tribunal because I read it's much tougher as the MQ? Now this isn't really important, but in the MQ I'm at the point where I was sent to kill Dagoth Gares.

What should I do? Should I start Tribunal and lower the difficulty if I find it too hard? Does it make sense to even start it now or is more logical to continue after MQ is completed?
I would love to start Tribunal especially because I never did it before...

Thanks for any advice you might provide!

No, do not start Tribunal until after you've fulfilled the Nerevarine Prophecy. Otherwise, it'll be too difficult without the Nerevarine and/or Hortator items.

It's more logical to play Tribunal after the main quest, storywise.

Okay, I'll continue with MQ then. I talked to the guy in Ebonheart and I think attacks should stop now.

There should be at most nine attacks by the Dark Brotherhood until you travel to Mournhold.