I’d just export all content from the site you want to keep. Then create a new WP instance, import that exported data, tweak as required.
Once the site is OK, make the new site active for the desired domain name (in your hosting, point the domain to the new site’s folder). No messing with databases required.
Added 31 July 2020
Another way: Use the Clone plugin from Automatic to export subsite. Install the plugin as superadmin. Then admin to the subsite and use the Clone plugin to back up everything.
Create a new WP instance (single-site), then install the Clone plugin there. Then import the backup you made of the subsite into the new site via the plugin. Make sure you remember the admin credentials, as everything on the new site will be replaced during the restore process.
Everything from subsite will now be on the new site including themes, plugins, settings and content.