Seeing some of the girlies here talk about their ideal personal hell of being a pony that is not allowed any recreation has got me thinking about how to handle that level of dehumanization where entertainment doesn't get to have entertainment. It's one of the primary ways I expect the extreme slavery dream to run into reality, if recent posting didn't tell anyone.
The kind of slaves who want that kind of extreme lifestyle tend to be just as autistic as the owners who want that giant amount of responsibility. Probably means a immense amount of videogames, something like a scrappy laptop that a slave could play on as a reward would likely be the first idea but I think that misses the point of why a slave is that way. They are kept alive by this extreme personal connection, even being in another room to clean it is mildly torturous to not spend every waking second in Master's presence, looking at a screen instead of looking at Master would drive home just how wrong it is to her value system that she is doing this, so instead simply hauling them up on your lap is the solution.
Might need a especially petite girl to do this long term, the slave diet can only do so much. Have her play a videogame that's been kept for her as a reward for good service, or even simply because Master ordered her to since her lack of humanity was beginning to wear her down and effect her service. Having a eye towards keeping her mind healthy and fit for service is essential for long term stability in keeping this basket of mental illness and servility around for your entire life.
Extracting some price out of her in response for your kindness would also assist in helping her little brain. Sitting on Master's lap is heavenly but is also a little like you're using Master as a chair, so it's only fair to have chair duties. Actual equivalent time wouldn't be the best idea, so perhaps she can make up the difference with doing exercises in the corner of the room, alongside footstool duty.