I have almost exactly what your talking about. I have a bulkhead drilled into the side all the way at the bottom of my sump. On the outside there is a simple ball valve to start stop flow. On the inside of the sump I have a 90 and a short piece of pvc measured out to where 40g mark is.
To do a water change I simply turn the system off, turn that valve on th sump which will drain the sump 40g into my floor drain. Then turn on a pump to pump back in 40g of new water to my sump. I do this every Friday and it takes me about 5 minutes and super clean and simple.i have less than $200 in my setup and I'm mad I didn't do this years ago. Hard water changes are a thing of the past.