Transmission fluid

I had the missus pour that last bit, say 1/2 cup at a time, then wait till it all flowed in, then repeat. "Full" happens gradually, wipe the underside of of fill hole clean, then wait a sec. If it spills over a little you're done. Splash a pinky to verify. As long as you've got the car level I would just replace the fill plug (with a new washer), and torque to spec. What that spec is with 5th gen you'll need to track down. Gen 3 is 28 ft/lb, 4th gen a bit more. 30 flt/lb should be plenty...

Also, not sure if it's been mentioned (probably has), but the 5th gen has a new fluid spec (in Owner's Manual); it's not Toyota ATF WS. For the old fluid Toyota cautioned to only use fluid from freshly opened bottles, fwiw.

There's a gen 3 transaxle fluid link in my signature, has some general tips that might be useful. On a phone turn it landscape to see signatures.

 

The hand pumps are cheap, and there are giant syringes also available.

 

I've used both in the past and they tend to be messy and fighting gravity from the bottom up. If you try to go faster or push too hard on the syringe, there a high likely-hood the tubing will dislodge from the fill hole. Been there, done that, big mess. Never had that issue, when filling from the top, but I suppose someone could tug on the tubing to dislodge it.
IMHO, filling from the top is less stressful and requires a lot less energy than fighting gravity.

 

I can see hitting full with fluid still flowing down. Plus lots of bending and moving when doing it yourself.
Also don't know accessible to the differentials on a Subaru.