Scan the site for any scavengers or pirates. (I've done 3 of these missions and not found any yet but you should always be careful when leaving a ship).
Insert power regulator into generator and power the base. DO NOT VENT ATMOSPHERE OF POWER BUILDING
Loot all other buildings in the settlement, recharging your suit from wall ports and venting atmosphere to remove risk of burning.
If you run out of space, book an apex shuttle to any location, instead of boarding instead manage inventory and deposit all items into the ship. The ship will depart in 2 minutes and refund your 100cr, and the items stored will persist like an account bank.
Return to power building, turn off power and take the power regulator
Quit to menu and re-join
Repeat from step 3
Because you never put out the fires in the PWR building the mission does not complete and you get to keep your level 3 authorization. Relogging will reset all loot and data downloads and you can just keep running laps. As long as you vent the buildings before you loot you should never take any damage. This will obviously net you a lot of engineering materials but also goods to sell at the bar for between 10-100k per looted item and 2k-20k per downloaded file.