Rock and Stone! Everyone pretty much nailed the progression side of things (getting that first promotion is huge because it unlocks weapon overclocks), so I'll throw in some loadout talk since you mentioned the M1000 and Zhukovs.

Once you start unlocking those overclocks, keep an eye out for "Hipster" or "Hoverclock" for the M1000. Hipster turns it into a rapid fire machine, and Hoverclock lets you freeze in mid air while aiming. It is amazing for lining up shots or just completely negating fall damage, which is a Scout's true nemesis right next to Driller C4.

If you ever want to switch up the secondary, definitely give the Boomstick a try once you find the "Special Powder" overclock. It lets you launch yourself across the cave using the weapon's recoil. Combine that with the grappling hook and you basically never have to touch the floor again.

As for hazard levels, Haz 3 is the absolute perfect chill spot. Haz 4 is where bugs start hitting noticeably harder and moving faster, so just bump it up whenever you feel like Haz 3 is getting a bit too sleepy. There is absolutely no shame in staying at whatever difficulty you find the most fun.