I'm not exactly sure what you mean by simple maintenance but I have commented on manufacture and modularity.
I'll review and link some of it here as the comments were really in the wrong place. I first found this board from there and didn't realize the structure at the time. It's a bit of a dump but maybe having it all close together will help.
The general idea is as above Dielectric Elastomers. They use electric fields to actuate. The good thing is they are made of silicon and other elastomers which of course you can get at any hardware store to experiment with. Here's some links on this stuff.
Here's a link to a good book on these called "Dielectric Elastomers as Electromechanical Transducers: Fundamentals, Materials, Devices, Models and Applications of an Emerging Electroactive Polymer Technology"
Dielectric Elastomers can also be used as sensors
A good page with links to 100 papers on Dielectric elastomers
A link to a SUPER video. Note that D.E. can be used for computer logic. This means that with a few lines we could control muscles and possibly feed back sensor(touch) data. Using serial lines vastly reducing cost. Just like SATA and USB drive interfaces are cheaper than the older parallel interfaces. Large numbers of wires are a pain in the ass, costly and troublesome.
With the right logic you could move some logic into the actuators themselves. I think actual human muscles have some logic in them??? An example would be if the waifu hits something it would reduce it's force in the muscle.
I describe above how you could make these.
The idea is to roll out thin sheets of this stuff like a round homemade taco shell or tortilla. You lay out a layer. Silk screen on the control lines, logic and add parallel lines of strong fibers and power/control lines sticking far outside the ends of the layer, fishing line should do nicely, maybe or likely have several layers on top of each other for different functions, add one or more power/control lines to the logic then you roll the thing up from one end. What you have is something that looks very much like a muscle with a thick middle and skinny ends with the fishing line stuck out the ends that are tendons.
Visualize each little muscle much smaller and shorter than the whole muscle but with long tendon and power/control lines.
Say you are building a muscle, the one on your upper arm that you flex to show how strong you are. You would build this out of a bundle of maybe 30 of these small muscles. You tie them together in series for length and add them in parallel (each offset from each other to stack smoothly).
All the fishing line tendons would wrap around anchor points in the bone
Once you have a muscle bundled together in a group you could in turn pack it in some sort of cloth covering to keep them all together.
Obviously you could remove these and replace individual muscles or the whole group.
As for sizes you could maybe have 3 or 4 sizes. The smallest would be very small such that it would still take 20 or 30 of these to make a finger muscle. This gives you redundancy and having small muscles in a bundle let's them move around realistically like real muscle.
"Muscles, Reflexes, and Locomotion" by Thomas A. McMahon
Some ideas. I also touch on skin.
I think the best way to make skin is to use that fabric called microfiber with a little bit of elastic fiber added in to make it stretch. It's super soft to the touch. It's used for towels and you could throw the whole waifu in the tub, scrub it off then drip dry. If it's smart enough it could do it itself.
The skin could be one big piece from toes to head. The vagina, butt could be pockets that go into a cavity. To connect them you move aside the abdominal muscles pull into the body tight and attach to a holder. Possibly be held in place with Velcro??? Reach into the top of the head for the mouth and ears. Skull cap could be removable. Wig, skull cap to cover???
If you connect the vagina and butt with the mouth the robot could drink water and wash itself out.
Don't forget the elastomer muscles in the sensual areas. They could vibrate and/or contract to do wonderful things. These parts could be removable.
I look at all these mechanical things, electric motors, gear boxes, noisy hydraulics, pnumatics and I can't imagine you could ever get the cost down on these things. They're great but they produce a lot of noise, they take a vast amount of machining resources and if they are 3D printed most plastic is fairly weak. If you use electric servos or motors you're going to have to have some sort of transmission and all of them will make a huge racket. You will have a grinding, thumping washing machine for a mate. Hydraulics and pneumatics are very inefficient and make a lot of noise.
I think the Dielectric Elastomers are the most fruitful path but...it's going to take a lot of experiments and work to get these to work.
Motors, pnumatics, etc., well you can cobble something together right now that will work but I expect it will never reach a satisfactory conclusion and will always be expensive, noisy and inefficient.
The D.E. made be a more difficult path but I could see it being extremely lifelike.
One last thing about skin. I'm opposed to silicon. It's messy, tears easily, collects dirt and everyone will know it's a robot. Why not cover it in micro-fiber that's easy to keep clean and super smooth to the touch.