For 1.5 model you
have to use 512x512 images.
Use rpaExtract tool to unpack images so you don't have to separate characters from backgrounds by yourself.
Its worth to try to prepare images manually and make few versions of each with different background, for exmaple Ash on black background and exactly same Ash on white background. If image doesn't fully fit in 512x512, you can try to prepare 3 versions of it (or lets say 6 with different backgrounds): fullbody resized down to fit in 512, cut top part without resizing, cut bottom part without resizing. Prepare decent text descriptions for each image so they gonna work with similar prompts later (descriptions can be generated automatically by DB, but it may not work good enough for your case).
Also it might be worth to use some anime focused model such as "anything v3" as a base instead of original SD1.5, and use Danbooru tags in your text descriptions (in that case learn about their tags before you write text descriptions).
Begin with small dataset and just one character, something like 30 images should be enough to get first results and decide if its time to add mode images / characters or if you still need to tweak training parameters.
If you gonna have more questions on this matter send me a private message.
But I won't be able to help you with particular training parameter values as its something you have experiment with (easily can take several days). I'm not an expert in style training and mostly train models based on real humans for deep fake purposes, not artworks.
And to keep it somewhat on topic.. heres my quick fanimation with Stan + Lena.
View attachment 2488768
hi res mp4:
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