I've already used an
Organization schema markup for my main site.
Can I use the same for my subdomain (which is a review site)?
Yes, you can use it as long as is the same company and that info is already present in the subdomain.
In addition, don't forget to use ratings & reviews schema in your subdomain.
Yes, You can. I think its fine with just mentioning the root domain there. Because you are describing 0r show a company and sub-domain is a part of that...
Google's John Mueller says that schema should generally be unique and implemented on a page which has the schema data as its main focus: https://www.searchenginejournal.com/google-structured-data-unique-page/239507/
It sounds like your main domain home page is the correct place for your organization schema. It shouldn't be reused on the sub-domain or even on other pages on the main domain.