Beautiful scenery!
I've had 265k miles when I decided to replace rear springs, put an order for original toyota springs that after 3 months waiting it was still back ordered, shipping from abroad was prohibitive as well as the crazy price for oem's, so I went with OME 865's, and added Air lift air bags. It stiffened up the rear a bit and lifted by at least 2.5in from my saggy originals, but settled after a few weeks. It rides OK, still not 100% on the compatibility between the spring rate and the OEM shocks (newly installed too). I'm thinking that a progressive spring might be a better option, but not disappointed either, just thinking out loud.
Maybe try first the air bags anyway? (if the unloaded ride is acceptable in your eyes).. I keep them with 6-8 psi unloaded, and when the family is loaded with a weekend trip load I pump to 15-17 psi using a bicycle pump in a couple of minutes. No leaks from mine for the last 1.5 yrs. Look for the threads on the install, really easy and you can still keep the original springs and save some $$ to use for gas and more amazing travels..