The partial drain/fills are easy - just remove the drain plug, let the ~4 quarts drain back in, and refill ~4 quarts into the plug at the back of a transmission. You can do it in your driveway without jacking up the GX. However it's not my favorite way to do it as 1) you co-mingle fresh new fluid with old fluid and 2) the pan isn't dropped and junk isn't cleaned out of it. It's like taking a shower and putting dirty clothes back on. But I think it's still better than leaving old fluid in there.
The better way is a full fluid exchange with a pan drop, which is only slightly more difficult (albeit messy). This is done from from the trans cooler in front of the radiator, along with a pan drop/clean and filter change. However I would absolutely not give this to just any shop - they need to understand how to do it properly (drop pan, clean pan, replace filter, refill pan, start engine and pump out 2 quarts from cooler while a helper runs the trans thru all gears, stop engine, add 2 quarts back in, and repeat til clean fluid comes out of the trans cooler). If you don't have a shop willing to take the time to do it right, find a friend willing to help you. It's maybe 2-3 hours of work and requires zero special tools.
As far as the overall yes/no - up to you. Only bad things can happen by running your trans on old fluid. If the trans is already past the point of no return then you'll need a new one anyway - depends on your desire the keep the rig and ability to have $4-5K set aside for a reman Toyota trans and replacement. The Blackstone test would help give you some certainty on how the trans is doing overall.