Does anyone know if there’s a keyboard sequence that’s equivalent to substitute in vim, specifically for replacing text on the current line only?

if you have enabled vintage package and in selection find,

just do escape, shift+v, ctrl+h, type in search pattern and words for replacement, then ctrl+shift+h for “replace” or ctrl+alt+enter for “replace all”

This is implemented here:

github.com/SublimeText/VintageEx