bp_after_setup_theme() is where BuddyPress loads the theme functions. If you want to filter actions in functions.php or buddypress-functions.php hook to bp_after_setup_theme().

add_action( ‘bp_after_setup_theme’, ‘bp_load_theme_functions’, 1 );

Source File

bp_after_setup_theme() is located in bp-core/bp-core-dependency.php