Any contacts in your phone that are entered as Account contacts (not Phone or SIM contacts) are already there for you to edit with an app you have - your web browser. Go to
Google Contacts and you'll see them. (Any contacts in the phone that are not Account contacts - turn off wifi and data on the phone, or turn off contact syncing - then just enter them on the page as new contacts, then delete them from the phone. When you sync again, both the page and the phone will have them as Account contacts.)
You're not "sharing" data. Google's cloud servers have billions of phone numbers on them, and no one has the time (or access) to go mining through them.
However, if you're really paranoid, get MyPhoneExplorer (free at the Play store). The easiest way to use that for what you want is to go to its Extras menu and choose Text Input. (You'll also have to add it to your Language and Input in Settings on the phone.) Then you can use your keyboard on your computer as the phone's keyboard.
Or, if you don't trust software, buy an OTG cable. (They're a couple of bucks on eBay or $40 if you buy one from Samsung - but aside from Samsung's cheap connectors, they're the same thing.) Then you can plug any USB keyboard directly into your phone.
