Cyanlabs ford sync 3.4 keeps asking to update, becoming less responsive

Ok, you need to get out of that build, since it’s has a bug.
Your rwdata partition is full, causing the issue.

You need to follow this guide and install the latest Sync3 version available → APIM Sync Settings Not Saving, VER_ERR08 Errors, Cannot Perform Master Reset or Upgrades/Updates (Solved)