Before uninstalling Azure ad connect , I invite you to read the following to check that you don't need any feature supported by Azure AD connect and not by Cloud sync:
Comparison between Azure AD Connect and cloud sync
Regarding your question , Could sync can co-exist with Azure AD Connect.
I invite you to read the following for more details about how you can migrate Azure AD connect to Cloud Sync:
How To Migrate To Azure AD Connect Cloud Sync
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