You can't directly add onto a custom_column via replace_metadata, you'd need add_to_custom_columns_settings for that. But since you also want to have different data on #mycollections, you'll need to make another extra entry where you'll add the value from category. Here's how I would do that:
Code:
add_to_extra_valid_entries:,category_modified
include_in_category_modified:category
replace_metadata:
category_modified=>House of the Dragon (TV)=>House of the Dragon
add_to_custom_columns_settings:
category_modified=>#mycollections
Note the comma before category_modified in add_to_extra_valid_entries.
Make sure Allow custom columns settings from personal.ini to override is check in Fanficare's settings under the Custom Columns tab.