I'm not an expert on frame damage, but if I had to be hit anywhere, I'd prefer where you were hit. I think rear frame is far easier than the front. YOu mentioned it was tweaked at the rear shackle. That's even further back and probably a much easier fix. I would imagine it’s tweaked at the weak C-channel part of the rear Taco frame?
As mentioned by others, I would use this an opportunity. A golden opportunity. Even if the truck was totaled, I would do everything you could to keep the truck. I don’t know how insurance laws work in your state. Sometimes they insist on taking the truck and not allowing you to keep it. But I would do everything I could to keep it. Even just for the motor/trans set up, if the frame couldn’t be fixed, but I just can’t imagine that it couldn’t be.
The trans is aluminum. There’s no surprise that it’s cracked and leaking, if it got hit. A new one could be installed by Toyota pretty easily if the insurance will cover it, as they should.
5400lbs is not bad, but it does exceed the Taco's gross weight by about 300lbs, which isn’t too bad, but if it were my truck, this is what I do with it.
1) During frame repairs, I would upgrade the rear frame, like Scott did on his rig.
2) Add the Alcan or Deaver springs. I went with Alcan and I personally love them. They are good people and if you given them proper measurements and weights, they will build them right. One thing to remember is that even though you will give them your rear weight of each wheel, you have a lot of weight behind the axle that needs to be accounted for. I would send them a picture and as much info as you can about your rig.
3) This isn’t mandatory, but I would consider strengthening the spring mounts on the axles. I’ve seen the Taco mounts bend with excess weight.
4) This also isn’t mandatory, but I would consider upgrading the rear axle to a full floating axle. I know there are several kits for old Toyota trucks. Not sure about the Taco axle though.
You’re plan to keep CG low is a very wise one. I would definitely keep your ride height stock. You might be able to add a thicker aftermarket anti-roll bar up front to help a bit, since articulation really isn’t going matter that much.
The one off road mod I would consider is a rear locker. I’m not sure if yours has the factory electric locker or not, but if not, an ARB air locker in the rear could really come in handy if you get stuck in the sand, especially since you aren’t winch equipped. I didn’t notice any sand ladders in your set up. Without a winch, but doing a lot of beach driving, that might be a good addition to your supplies. They aren’t cheap, but they are light enough usually that you can through them on top or the side of the camper.
I'm sorry this happened. But I really think it's fixable and you could be back on the road in a few months. Good luck.