Man, I feel like the 'Wizardess Heart' fanfic scene has been scattered since the original visual novel community kind of faded. Most of the old dedicated forums are ghost towns now. I've had my best luck digging through the 'Otome Game' tags on Archive of Our Own—some writers will tag stories with the specific character's name even if they don't use the fandom tag directly. The selection isn't huge, but there's this one slow-burn Alistair/Rosalind fic that absolutely nails his quietly possessive vibe.
Tumblr is a mess for searching, but I found a couple of active writers by looking through old 'wizardess heart' blog rec lists and checking if those blogs are still reblogging fic snippets. Wattpad's algorithm buries anything that isn't massively popular, so I wouldn't start there. Honestly, your best bet is joining a general otome game fan Discord; people in those often have private Google Drive folders or Carrd links with fic archives for older titles like this one.