Hello,
1) Follow the steps described here https://xstore.helpscoutdocs.com/article/100-how-to-speed-up-my-website to speed up your site
2) Ok. The Elementor page builder is a very handy builder.
3) Don’t understand your sentence. ” They could check my Dashboard” – who are they?
4) Do you plan to use newsletters? If you don’t then you can just disable that plugin. If you want to create newsletters but Mailchimp is not suitable for you you may look for any other plugins that will be more suitable for your purposes.
5) 500 error means that some fatal error appears on your site. Provide us with server error logs and also with WP Dashboard and FTP access to check this in more detail.
Regards