Set your boot disk from within mac os, then check ShowPicker property in the config.plist, to automatically boot without user interaction you need it set to false. Otherwise, if you want the picker (what you call boot manager) to always show, set it to true, but change the timeout value. I recommend all to read opencore documentation: https://github.com/acidanthera/OpenCorePkg/blob/master/Docs/Configuration.pdf   All is well documented.