(content) Propagation + website in multiple languages
Hopefully someone has a better answer than me, but I’ve been in the same situation recently and the way I eventually did it was to output the field for each page in Json using element api, then create a new field that does allow different blocks per locale/site and re-import the content back into that field per locale using feed me.

I left both fields on the control panel for a period of time so the client could check everything was correct, then moved the templates over to use the new field.