The difference here is splitgate stuff was (afaik, only played a tiny bit) all cosmetic. The helldivers passes actually do have different weapons and armor so it does change how you play, but most weapons are viable, especially at lower difficulties so try out everything and see what you like. But again, absolutely no time limit on anything in the game except the super credit store (which is just extra armor and it rotates through a normal cycle) and the missions themselves.

Just some very general tips so you aren't too lost. Enemies can call for help. Bugs rear up and bard orange pheromones into the air. Bots shoot a flare. You can kill them before they finish calling for help. Some enemies have armor. If your shots bounce off, try hitting a different spot or use a gun with armor penetration. 380 barrage is useful for big enemy bases, but tell your teammates. It hits a BIG area. Difficulty only changes what enemies spawn and how many. No changes to health or damage.

Also even the earliest unlocks are usable at the highest levels, so don't worry about using "noob" stuff.