use theme.json along with Blocksy Customizer Content Width settings

blc-layout-themejson-filter.php

<?php
function blocksy_layout_update($editor_settings, $block_editor_context){
	// default values:
	$editor_settings["__experimentalFeatures"]["layout"]["contentSize"] = "var(--has-wide, var(--block-max-width))";
	$editor_settings["__experimentalFeatures"]["layout"]["wideSize"] = "calc(var(--block-max-width) + 260px)";
	// maybe add offset values:
	if($mods = get_theme_mods())
		if(isset($mods["wideOffset"]))
			$editor_settings["__experimentalFeatures"]["layout"]["wideSize"] = "calc(var(--block-max-width) + ".($mods["wideOffset"]*2)."px)";
	return $editor_settings;
}
add_filter( 'block_editor_settings_all', 'blocksy_layout_update',10,2 );