I didn't really like what most of the other sagging breasts models for anime were doing, so I compiled sample images, cleaned them up, and have started working on my own LoRA for this subject. New version for illustrious. Note that the new version doesn't tend to enlarge breast size, and it tends to adhere to prompts better. Since it is illustrious, rather than xl, you might experience degradation for non-animated art. It sometimes will really try to make nsfw though, so you may need to put sagnips in negative, as well as other nsfw terms.
Good weights are between 0.5 and 1.0, depending on what you're trying to do. It's actually quite versatile in that regard.
Saggingbreasts is the main keyword to be used in all the prompts. Beyond that, sagrest (especially in conjunction with breast rest), sagnips, sagcrawl, and sagclothes each have their own environment to be used in. If nipples will be visible, add sagnips. If the character is wearing clothes that will support the breasts somewhat, add sagclothes. Sagrest should be used when the character has their breast or breasts resting on an object. sagcrawl should be used in conjuction with all fours.