TRANSMISSION SERVICE (not a flush) purchase a pan gasket and filter, drain and drop the pan replace filter install new gasket on pan reinstall pan and just add the amount of ATF that was drained for maybe 5 quarts. Done.
Now your transmission should last as long as your engine. You will feel an immediate difference (in some cases noticeably better gas mileage).
A couple more suggestions.
*TRD supercharger for around 4k, or…
*Bullydog tuner for about $400 and can add 50-100 horsepower or so.