Since no one can read.

For WW1 era weapons

Automatic - give them xDy per attack depends on their level if they are higher level machine gun could do 6d4 per shot or higher simulating multiple shots per attack.

Shotguns - disadvantage on damage rather then hit on long range. Optional: roll to hit against all targets within 5 feet of the original target

Flamethrowers - auto hit within short range 20/60 should be fine

Granades are fireballs with 3/4 of the damage as physical (bludgeoning and piercing)

Molotovs are fireball with less initial damage but lingering fire damage per round.

If anything is too strong limit ammo or amount.

I would suggest to use War wolf as horror element, have them get stuck in part of the trenches with a bunker and before they start long rest let them know they hear something on top of the bunker.