All of the questions you asked are specific to your particular WordPress website and the plugins and themes you’re using. I can try to help, but I’m not an expert on every WordPress plugin and theme out there, so there is only so much I can do. If specific themes or plugins are causing issues, perhaps ask their creators for more information.
The first point tells me you’re using Elementor, because “starter themes” is not a WordPress feature.
All I can say is that Elementor is a bloated best that doesn’t run well on our hosting. I strongly recommend to avoid Elementor and any theme that relies on it, and use something more lightweight. Or be prepared to pay for the convenience that Elementor provides through your hosting bill.
Whether you need to edit existing CSS or add new CSS depends on the code that’s currently there.
If you’re going to tinker with CSS, you should know how to work with CSS. If you don’t know, then you could either try to learn how this works, or just do trial and error.
Deleting everything there is probably not what you want. But it’s your site and your design, so the only person who can definitely tell what should be there and what shouldn’t is you.
We do not recommend the use of backup plugins. Backing up an entire website through PHP codes puts a lot of stress on the server, which can trigger suspensions.
If you want to backup your website, we recommend to just do it the old-fashioned way by backing up the files and database:
As for where your particular plugin stores their backup or how to restore it, please check the documentation of the plugin you’re using. Different plugins work differently and have different options.
We don’t cache anything, and neither does WordPress. if you use a caching plugin, then that plugin should have an option to clear cache.
As for the bad speed, my first thought would be to trim the plugins you’re using. It sounds like you crammed your website full with big, complicated plugins. That will definitely slow down your site.
If you don’t trust the built-in update checker, then check the version you have installed against the latest version published by the theme creator.
Please understand that we are your hosting provider, not your website administrator. It’s not up to us to teach you how to use WordPress, or to fix your website for you.
We can try to help point you in the right direction, but please understand that most people here (myself included) are not WordPress experts.