Excellent progress Anon.
>Any low cost alternative ideas?
Use triangles. For example run two cross-braces on the thighs each (one on the front, one the other angle
on the back). Repeat for the tail's joints, and the lower leg's joints. So for the front thighs, you'd run one diagonally from the inner crotch, down the thigh, then attached to the outer knee. Do the exact opposite angle (outer butt to inner knee) on the backside. Mirror on the opposite side (the other way round). Do the same thing for the other 4 remaining 'limb' parts.
That should help with individual limb parts rigidity. You now can think about stability
(not the same issue). For that, you can think about angling the two legs outward slightly to form more of a 'bow-legged' tripod form in space. Slightly awkward ATM, but in the future as you begin to turn her into a real robowaifu, she'll have active balancing mechanisms available -- same as we humans do.
Those stabilizing gyroscope cubes we've talked about here for instance, might be a feasible way to improve her balance and stability. (>>5645
Lastly, as you move forward with her design, attempt to locate as many high-mass items (batteries & motors for example) inside
her pelvic volume. Not
in the higher, upper volume of the chest/head. I believe you'll quickly discover this engineering approach will assist you in many ways with your designs, not the least of which will be the balancing your robowaifu.