2011 SR5 337k miles Transmission

Recently got this 2011 4Runner and have put about 10k on it. I have an appointment tomorrow to get the transmission looked at. I’ve been told there is no way to even look at the fluid since it is a sealed unit. How should this be done without causing a problem? Should I get a partial fluid change or just a top off? Or if the fluid is burnt should I get a complete fluid change? It is shifting a tad rough and I have PTSD from shit breaking. My work truck (not a yota) for my small business has blown two head gaskets and had to have a transmission rebuild in the last 3 years so I’m just worried.