You didn't answer my question about your problem with non-critting MM skills.
The answer from Steam sounds reasonable, but mind my very limited experience with Marksman because I don't play one. Nonetheless, Scoundrel seems like a good synergy ability for a Marksman in general and for you especially, since it gives you much to do off-combat, like sneaking around in settlements or scouting ahead in the wilderness. I don't want to spoil too much, but a Scoundrel is also well suited for the first main quest that involves much investigation – and who would be better suited for that?
As for your xbow build, I did a (quick) web search and found these:
Maybe you didn't know them already. But then again, I can't evaluate them for lack of Marksman / ranged weapons experience.
