I would not bother with the oil pan unless you are contemplating frequent track play. For a street driven car it is unnecessary. If you are doing it though, now is the time as you might consider changing every filter, every lubricant and every fluid on the car as well as attending to a belt swap if required. You could skip some of that if you have the service history but you don't so just do it all.
I bought my car new in 2017 and that's exactly what I'm doing over the winter, now that it's eight years old. There are some "sneaky, out of mind" fluids too. For instance, there's a separate lubricant in the supercharger that IIRC is said to be "lifetime." Is it lifetime? Is anything? I don't know but I will be checking/changing that as required along with all scheduled maintenance like antifreeze, brake/clutch fluid of course, transmission, etc. Since you don't have records, then I think after eight years the car deserves a fresh start. Do it all in whatever order you can manage but I think it's best to just bite the bullet and get it done in one shot.
As for getting the decat? Great mod, do it along with that major service. It uncorks the sound and you'll want that. These cars are well worth the investment and are valued as much by condition as age. Toyota tough, have faith! This will pay off in peace of mind for thousands of miles!