Even with different colours, this is a mess:

My suggestion: if I put the mouse on one of the quest markers and I hold some key like Control, the rest of quest markers become translucent or disappear.

@wodim You can use the tracker to "focus" a quest/objective, which will fade out all others. With the next release the objectives for one quest will also be highlighted when you mouseover them

With the next release the objectives for one quest will also be highlighted when you mouseover them

Oh, then that's it. Thanks.