I got lucky.
I was on good terms with the outlander union of this playthrough (I'd been sending them drop pods full of packaged survival meals and whatever garbage raiders drop until they became allies), and fortunately, after a few weeks of camping in the hills outside my own colony, they started sending backup to assist with the mechanoid raid that never ended. The first couple got wiped out, but the third managed to kill off the last few mechanoids.
So I waited for a lull in zombie activity, then quickly packed everything I could carry back from my camp and headed back to the colony.
Fortunately, as I said previously, the nymphs I recruited were not as useless as typical nymphs - I made sure I got ones that could do construction and item crafting (the "Curious Nymph" starting background). My colony had lots of gaping holes in its walls and destroyed power runs and doors from fires that spread throughout the complex, but because most of my base is done up in stone the fire spread was relatively contained, but for construction and repair I actually had a fairly complete workforce.
Thank goodness I spent a lot of the game up to this point converting every drop of excess food into packaged survival meals because my farms were all completely blighted and needed to be burned down and started over. The animal pen didn't get rebuilt in time so some of my animals got away, but there's not much I could do about that. I'll have to re-tame or buy new animals, but in the meantime I think I should be able to keep everybody fed, and morale is climbing now that people aren't in mass barracks with sleeping bags any more, and can actually sleep in nice beds (with their spouses/lovers where applicable).
I got all my main entrance turrets back up and running too, and managed to restore the power grid with some hunters laying down covering fire for the nymphs rebuilding the electrical cables out to my geothermal generators. All my backup chemfuel generators got trashed, as well as one of my hydroponics rooms, but they're lower priority to walls and defenses, which still need some work. Thankfully the main crafting area, dining hall, and hospital were completely untouched, other than being "technically outdoors" because of doors burning down, which led to some supplies decaying over time until they disappeared... but that was mostly cloth and junk weaponry.
So, we're starting to claw our way back... I'm sure Randy has something special cooking up for me soon.