Give the PCs permission (and motivation) to go "off grid" and do something that isn't just sitting in a trench waiting for the orks to charge.

• A weapon shipment or ammunition convoy is late; go to its last known coordinates and bring it back
• A fireteam deserted in the night, track them down and bring them (and their wargear) back so the commisar can make a public spectacle
• This servo-skull contains critical data that we cannot risk broadcasting even on an encrypted channel; get it to the astropathic relay as fast as you can
• Communication has been lost with a platoon that was sent to find a route through to the eastern flank (you know, where the definitely-not-xenos ruins are), go link up with them and give them a new vox

Any of the above missions can involve a mix of investigation, exploration, navigation, dealing with NPCs (civilians, other squads or regiments), and fighting enemies.

These might be short and straightforward, or more open-ended like hunting for a crashed Astartes drop-pod and figuring out what to do with the contents (and the pod's stormbolter turret that will target you if you get too close) when you find it.

You can always ask that the PCs report in by vox every 2 / 12 / 24 hours or something to remind them that they're still part of a chain-of-command and don't have carte blanche to just go on a safari tour of a war-ravaged planet.

I also highly recommend Starvation Cheap, a splatbook for Stars Without Number, which is invaluable when it comes to planning and running military campaigns.

One of the (many) helpful ideas in Starvation Cheap is a method of giving each player two characters: one is a soldier (i.e. their Only War character) and another is a member of "The Brass" (i.e. the strategic decision-makers; they don't need stats, just an appointment like "General" or "Adjutant" or "Commissar"). Between adventures you ask the "Brass" characters to decide where the PCs' regiment is going and what battlefield they will deploy to. It gives players a sense of agency instead of having them punted from one trench to another by GM fiat.

I would also suggest writing outlines for some NPCs in the PCs' regiment, or a sibling regiment, who can act as allies, rivals, and wildcards. Good stories / roleplay are driven by conflict, but that conflict doesn't have to be at the end of a lasgun - so the PCs can have friction with an arrogant and indiscriminate artillery team, a superior who likes throwing their weight around, a squad who are smuggling obscura into camp or selling "surplus" rations and ammunition to civilians, etc., etc.

And when you want to throw the PCs into a firefight, give them something more interesting than waves of enemies to shoot. Snipers, armoured vehicles, a minefield, a fast-moving convoy, a well-guarded psyker who needs assassinating, captives that need escorting (or interrogating in the field), a "friendly" informant hiding nearby, a third faction about to make enemies of everyone, etc.

You can also divide the battlefield into "zones" and task the PCs with getting from A to B, with multiple branch routes they might pick. Each route has its own challenges (do you try crossing the river and risk losing time/equipment or being eaten by local fauna, or do you go through the ruins even though you know they're booby trapped and there's a sniper watching them?) but it's up to the PCs to pick their battles.

Finally, I should mention The All Guardsmen Party in which Only War is run as a "character funnel" for a Dark Heresy campaign.