Hires fix or Tiled Diffusion (+ADtailer) would give good results.
(If interested in upscaling settings, see : https://civitai.com/articles/3588)

Recommended weight: around 0.7-0.8
Triger Word: hks,
Sample prompts:

Normal:

hks, hoshikawa sara, blonde hair,  (side ponytail:1.2), multicolored hair ribbon, ahoge, x hair ornament,  heterochromia, large breasts,white tank top, crop top,  off shoulder,open jacket,collarbone, choker, sleeves past fingers,navel,blue shorts,

School uniform:

hks, hoshikawa sara, blonde hair, twintails, ahoge, hair ribbon,red ribbon, heterochromia,  breasts,school uniform, brown jacket,  white shirt, collarbone,  bowtie, pleated miniskirt, open jacket, black thighhighs, garter straps,

White uniform:

hks, hoshikawa sara, blonde hair,  very long hair, two side up, ahoge, hair ribbon, black ribbon, heterochromia,breasts, military hat, peaked cap,white jacket, white shirt, white shirt, long sleeves, black necktie, black gloves, belt, thigh strap, brown pantyhose, cloak, white cape, 

Red sweater:

hks, hoshikawa sara, blonde hair, alternate hairstyle, very long hair, ahoge, hair ribbon,black ribbon, heterochromia,breasts,off shoulder shirt, red sweater, long sleeves, off shoulder, bare shoulders, collarbone, choker, red bow, plaid shorts, black pantyhose, 

Pink cardigan:

hks, 1girl, solo, long hair, blonde hair, hair pinned back, forehead, single braid, braid over shoulder, x hair ornament, heterochromia, large breasts, official alternate costume,pink cardigan, puffy long sleeves,  sleeves past wrists, open cardigan, frills, white bow,  open_clothes,  white camisole,  collarbone, midriff, navel,  jewelry, pink shorts, frilled shorts, necklace,  white socks,