Do I 'have' to switch to Google Drive? Security implications?

1. Yes, Backup and Sync will be gone, according to this tweakers.net article by Oct 1 2021
2. As far as I have seen, none. The login is configured when you first install Drive, settings are retained so when it's running it autologins. If you shut it down or logout, no sync should take place.
3. I've never used BU&S so I don't know how it was configured, but Google Drive creates its own folder or partition. It doesn't try to reach outside of that.

All Google services are available with all Google accounts, whether or not you use Drive. Things like the Office suite and Calendar are web-based, i.e. run in Google cloud, nothing's installed on your hard drive.

Just for a heads-up so you won't get surprises - Google Drive default size is 15GB, and AFAIK it's shared with all Google services you use.