Astartes face the same hazards as everyone else when it comes to long distance warp travel.
Um... no. They face the same hazards as everyone else who is mentally hardened/fearless and rides in one of the best ships available to the Imperium, probably sporting high-level variants of the Warpsbane hull and other gizmos along with the top Navigators of the time. That's a bit less hazard than "everyone" everyone else.
Admittedly there was an encounter in the Segmentum Obscurus with what are suspected to be Tyranid bioforms in M35 which has since been referred to as the remnants of Hive Fleet Tiamet. That said I'm not so sure they would need Tyrand fighting specialists due to that one isolated incident some six thousand years before present time (within the setting), and it certainly doesn't constitute an immediate Tyranid threat to that portion of the Imperium. As far as genestealer cults go, they've been rampant for a while so I would imagine many Chapters would be well versed in dealing with them.
I'm sorry but it just seems like you're grasping at straws to me, but you have a right to your opinion.
As has already been noted, there are Lictors in or around the Calixis sector - they are noted in Creatures Anathema. Lictors mean Tyranids somewhere in the future. Assuming you're a caring Inquisitor who doesn't want to see his sector become bug-lunch, who ya gonna call?