I'd say that Titus should be rather part of Grey Knights than Deathwatch. Deathwatch is fighting xenos while GK fights with daemons. The main threat in SM weren't the orks but daemons and this warp artifact. In my opinion it makes more sense but that's only what I think

Technically you are correct Grey Knights would fit the bill as they are the knights militant for one particular branch of the Inquisition, however in the context of the story it would have made more sense for him to be a Blackshield and assigned to Deathwatch.
I feel it would have been easier to craft a story where Xenos (Dark Eldar, maybe?) were using Warp energy/relics/beings to further their own ends.
Titus isn't quite at Grey Knight level of power - besides, he's not a psyker, he's something else. Something new.

Cool, badass, non-smurf ultramarine?

I second that.

No, I don't think a cool, badass, non-smurf ultramarine is possible and that's why Titus decided to start his own chapter rather that deal with the ultra-smurfs

I was actually googling for 'space marine 2' a few years ago in the hope for some news or leads when I accidentally landed on the announcement of Eternal Crusade that had just happened that day.

thats awesome, I was looking up Dark Millennium on the day EC was announced, high five for finding out on the same day

Don't play with my emotions, Sega.

Apparently being an adult isn't old enough to view the site.

Sega you play with my soul like Slaanesh plays with little kittens ..

Honestly I still play the campaign once every year or so.
Would not mind a sequel at all!

But for the love of the gods let us choose a chapter...