Forgot to mention that for this to work the variables in .env will have to be loaded in the bedrock applications config file (site/config/application.php or the environment specific config to try out things).
#site/config/application.php
/* Multisite */
...
Config::define('WP_ALLOW_MULTISITE', env('WP_ALLOW_MULTISITE'));
Config::define('MULTISITE', env('MULTISITE'));
Config::define('SUBDOMAIN_INSTALL', false);
Config::define('DOMAIN_CURRENT_SITE', env('DOMAIN_CURRENT_SITE'));
Config::define('PATH_CURRENT_SITE', env('PATH_CURRENT_SITE') ?: '/');
Config::define('SITE_ID_CURRENT_SITE', env('SITE_ID_CURRENT_SITE') ?: 1);
Config::define('BLOG_ID_CURRENT_SITE', env('BLOG_ID_CURRENT_SITE') ?: 1);
...