She was feeling worryingly possessive, like she had a duty of care as Spike’s principal bruiser
Love that expression "duty of care".

Everything they’d done was like a bond between them, heavy as the anchor chain her dad had given her to hold when she was twelve
Yeah, a real attachment that she might have to acknowledge...someday.

“It just is.”

Brilliant how real, painful and hopeful you painted the possibility of a grown-up Spike and Buffy.

To repeat -
a wonderful story!