So, the only way to Increase Load Capacity for Rear Coil Springs is make the Coil Longer/Higher.
EXAMPLE: I Can't keep OEM height and Increase Heavier load.
No increasing Diameter of Rod used to make HD Spring Stronger Stiffer and still keep at OEM HEIGHT.
SPECIFICALLY THE Coil Springs alone

air bags (Air Lift 1000), as an option?

Theoretically you should be able to increase the load rating and not the height. Problem is finding a company that makes this coil. Dobinsons has one that is 220 ish vs the factory 190. Listed as zero to little lift. You might still get 1/2” though

You can either increase the thickness of the spring or the number of coils (while maintaining the same height) to achieve a greater spring rate.

Here's an amazing chart that I found somewhere I don't remember, but someone listed most of the aftermarket rear springs for comparison. Keep in mind the 'spring height' listed is unloaded. So even if two springs were the same height unloaded, if one had a greater spring rate, then once they were loaded with the truck's weight, there would be a height gain because the greater spring rate reduced compression.

Attached Images

4Runner Spring Comparison.pdf (45.4 KB, 69 views)

this is incorrect. the only way to actually safely increase payload capacity without serious modifications is to reduce weight. adding air bags or suspension will help you handle your payload capacity better - there is no argument there but it won't increase manufacturer payload capacity. increased payloads need to be able to be properly handled by all the components of the vehicle (brakes, tires, drivetrain, frame, cooling, etc.) not just the suspension

it's often said that adding suspension upgrades and airbags will increase payload capacity. I get the rationale here "if my truck handles 1700lbs of bricks (max payload) better with suspension upgrades then I can add even more bricks until it handles as bad as it did before the upgrades. I must have increased my capacity" that is false. you have just improved the truck's ability to handle it's max payload capacity. very important difference

This.

We actually chose our gear based on this premise even though we're running 150lb over stock front/700lb over stock rear.

Minimal winch bumper insert
relatively light 8k winch
Synthetic winch line
Aluminum low-pro roof rack
Plate steel rear bumper with swingaway, with the swingaway now removed as we felt like we were far too close to our GVWR - live and learn.

Dobinson's would be my recommendation. Very solid suspension so far for us, and @Crikeymike is great to work with. I'd call ExitOffroad and talk to him about your goals (He sells Dobinson's gear)

Not sure if this answers your question, but I have variable weight dobinsons 3" lift springs in the rear, and get barely any sag when towing. I havent towed anything crazy, but have hauled furniture on a car-sized trailer, jet skis, and have loaded the cargo area with 1k lbs of pavers.

These coils here are right close to stock height, but a stronger spring rate (220lb vs about 190lb stock), so you'll be increasing the spring rate without changing the lift height.

Dobinsons C59-323 Stock Height Rear Coil Springs - Exit Offroad

Eaton Detroit Spring will make any custom spring you want.
Might be worth contacting them
EATON Detroit Spring - The leading manufacturer of leaf and coil springs for the street rod and restoration industries

A nice option might be the adjustable load leveling rear spring kit from Eibach. I don’t want to lift but with the weight of RTT and awning I have some sag, so this should bring it back to the stock height.

Google Anvil International Spring supports. You'll see many different spring options. But those are piping supports not vehicle supports. You can get springs made in any K factor which is stiffness in lbs per inch. So yes, you can get a stiffer spring made for the same ride height. The problem is getting a spring made that fits your vehicle, has the correct total length, and a higher stiffness rating.

But as other people have stated, it won't make your car able to carry more load. I feel your pain because I have a sedan that is tuned for comfort instead of sportiness and I would love to stiffen the suspension and keep the stock ride height.

Yes, I need STRONGER/STRONGEST SPRING RATE.
PER HEIGHT.
I know you cant exeed max GVW
There has to be some stronger steel or larger Rod Diameter Stock to produce what I want.
It should just mean make spring higher to make less sag when you for example Tow