I tested your configuration and Spotify Connect started automatically on the device. I made the following settings in the Spotify plugin: Autostart, username, password, output device, automatic audio switch. I set Squeezelite to the same output and activated the autostart there as well. Both services started automatically after a restart of the device.
If you use the Spotty plugin for the Squeezebox server, you don’t necessarily need our Spotify Connect plugin. You can set Spotify Connect Players in the Spotty plugin too.