Welcome to the PSR Tutorial Forum, Elia!
Current Yamaha arranger keyboards always have the same Voice settings immediately after starting, and ACMP is off. These start settings cannot be changed by the user. However, the keyboard remembers the last loaded registration bank and loads it again the next time it is started. However, no registration (buttons 1 - 8) is loaded from this bank yet.
If you want to press only one button to load your preferred settings after turning the keyboard on, you would have to make sure that your preferred registration bank is loaded before turning it off.
By the way, please note that the STYLE checkbox must be ticked to memorize the LEFT Voice settings in a registration. If only the VOICE checkbox is ticked, only the RIGHT Voice settings are memorized, i.e. the current LEFT Voice settings (including the PART ON/OFF status) would then not be changed by that registration.
Best regards,
Chris