Put the contents below in the file /Library/Displays/Contents/Resources/Overrides/DisplayVendorID-1e6d/DisplayProductID-76e4
,
create the folder/file structure as needed. Requires root permission. Reboot and your display should now have a 75hz option for 3440x1440
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>DisplayProductID</key>
<integer>30436</integer>
DisplayProductName