An important point here is that the situation in your home determines what you will have to do. Ultimately, the key is to look at the situation and devise solutions for your unique problems.
I've already described what I've done in our new home. Let me contrast that with a description of the arrangement in our former home, an older structure constructed around 1950.
The home faced north, with the kitchen in the southwest corner, and the bath on the east side. The main sanitary drain (cast iron, of course) ran down the eastern wall of the basement and out the southeast corner to the septic tank. There was a secondary drain (galvanized steel) from the kitchen that ran along the southern wall of the basement to join the main drain just before it left the house. The laundry was directly underneath the kitchen, and had a separate drain to a dedicated drywell outside the southwest corner of the house.
I built the darkroom in the southeast corner of the basement. Because the basement had a tendency to be damp, I framed all four walls and and lined the darkroom with sheet rock rather than use the basement walls for two sides of the darkroom. That meant that there was a space between the darkroom wall and the basement wall - a few inches on the east side, and about a foot on the south side. There was a cleanout in the main drain just before it penetrated the basement wall that would have been hidden behind the darkroom wall - to provide access in the event we ever needed to get to that cleanout, I framed in a removable section of the wall in front of the cleanout.
There was also a basement window on the east side. I was concerned that if I merely framed over the window, there was a risk of something breaking the glass and not being able to get at it to do a repair, so I first removed the window and filled the space with cinder blocks.
Initially, I used an ordinary sump pump in a reservoir (a 5-gal plastic pail) that exhausted via an old garden hose that I draped along the southern wall of the basement and dumped into the laundry sink.
This was an older home, and eventually the pipe from the laundry sink to its drywell clogged. Neither the plumber or I could open the line, so I had the plumber install a reservoir and pump to lift the graywater from the laundry sink up and into the kitchen drain.
It didn't make a lot of sense to have the graywater from the darkroom collect in a reservoir in the darkroom, and then be transferred to a second reservoir at the laundry area. So I purchased a "saddle tee" fitting of the appropriate size for the kitchen drain pipe. A saddle tee is simply a fitting that clamps around the outside of a pipe, and that has a rubber gasket at the area where the liquid transfers into the pipe. I drilled a hole in the galvanized steel kitchen drain and attached the saddle tee - this was done in that one foot space between the south darkroom wall and the basement wall.. I used a shorter length of the old garden hose to join the sump pump to the saddle tee. I also used a check valve to prevent backflow.