How can I keep the interior light off when the rear lift gate is open?

 

If it is the only component wired to its particular fuse (which is unlikely), you could pull the fuse.

The simplest solution may be to just pull the bulb out of the overhead assembly.

Some SUVs have an on/off switch for that assembly. My 2013 Subaru Outback 3.6R does. Not sure about the RX350.

 

I leave my lift gate open in the garage, sometimes for a whole day. After some time passes the interior lights turn off automatically, I think it's like 20 or 30 minutes. If you're worried about draining the battery, it's not going to happen.