So yes, unless you have some other page-rules or settings breaking wp-admin, you should not need to set any rules for wp-admin. Default Cloudflare functionality will work, including “standard cache” which caches static files (js, css etc) on Cloudflare.
This question is a bit unclear. First of all, it’s easy to misinterpret the “cache everything” page rule. Standard Cloudflare caching (without “cache everything” page rule), will already cache most assets (images, css, js). With “cache everything” page rule, you are instructing Cloudflare to also cache HTML (pages) output. However, even with this rule enabled, unless you have an “Edge cache TTL” rule or “cache-control” headers (from your origin), html documents will not cache anyway. Likely this rule alone does little or nothing.
In regards to your /wp-content/ directory, that should only be static assets, which Cloudflare will already cache. Setting “Cache everything” rule here will have no effect, and no it will not prevent Cloudflare from dumping them on a regular basis. Unlike other CDN services, Cloudflare decides to keep an item cached or not based on some internal mechanism. When you set edge-cache TTL or user cache-control headers, you are only instructing Cloudflare the maximum storage time.
For Wordpress, default Cloudflare settings are normally sufficient, and page-rules should not provide any advantage to Cloudflare caching. Cloudflare will cache static assets, and provide some basic protection to your website.
The exception is if you want to achieve page-caching in Wordpress, which can improve html loading times drastically, since WP is relatively slow to output pages. It’s dodgy business caching html pages, and you would need to create page-rules to bypass this in wp-admin/wp-login, and then have a rule to “cache everything” + either “edge cache TTL” or set cache-control headers from your server. Then you would also have to deal with the fact that Cloudflare is caching your pages, so any changes you make to the website are not immediately reflected, until you “purge cache” from Cloudflare dashboard.
To conclude, unless you want to use page-caching, there is not much you can do in terms of Cloudflare page-rules to speed up your website any more than it is with default Cloudflare settings. Many things you could do from your WP website of course, like use a caching plugin and serve public css/js scripts from a public CDN like cdnjs.com or jsdelivr.com.