Redirect entire site except wp-admin

You can use the site relocate to redirect everything to the same URL on the new site.

https://redirection.me/support/site-options/

If you want it to the home page you will need to create some custom redirects to redirect everything, but have a ‘do nothing’ for any URL you don’t want to redirect.

https://redirection.me/support/redirect-actions/

https://ww.wp.xz.cn/support/topic/redirect-all-pages-to-new-domain-except-admin-or-wp-admin/