Greetings, folks.

Project Brief:
Current Setup: site.de is powered by WordPress, utilizing Cloudflare's Pro Plan with SSL provided by Cloudflare.

Objective:
Transition the site to the site.com domain while maintaining SEO rankings and traffic flow.

Procedure:

Integrate the site.com domain into Cloudflare, ensuring SSL is active.

Clone the website and update all instances of site.de to site.com in the codebase and database.

Deploy the cloned site to site.com and verify its functionality.

Implement a redirect strategy:

Modify the htaccess file with the following rule:
RewriteEngine On
RewriteCond %{HTTPS_HOST} site.de
RewriteRule (.*) https://site.com/\$1 [R=301,L]
Update the robots.txt on site.de to reflect the new host as site.com. Seeking input on the necessity and rationale for this step.
Consider using Cloudflare for a 301 redirect to simplify the process and avoid direct htaccess modifications.

Update the new domain in webmaster tools for Google.

Does this cover all bases, or is there something I might have overlooked?

Appreciate the insights and assistance from everyone in advance.

While your plan covers the basics, it's a bit naive to assume that a simple domain switch won't impact SEO. Redirects can sometimes lead to a temporary loss in rankings, and the search engines might take time to fully index the new domain. The htaccess method is outdated and prone to errors; Cloudflare's page rules are more reliable and performant.
Also, updating the robots.txt on site.de is redundant if you have a proper 301 setup. Don't underestimate the importance of updating all backlinks and internal links to reflect the new domain, as these are crucial for maintaining link juice and authority.

To prevent any redirection hiccups, ensure your local server's domain name mirrors your actual domain name, complete with both HTTP and HTTPS protocols, with and without the 'www' prefix. If they're not twins, make them so.

Next up, double-check your paths to ensure they're not leading you astray. You wouldn't want your users landing in a 404 wasteland.

Now, if you're migrating your site to a different server type, say you're moving from an Apache local server to a real server with nginx php-fpm, be prepared for some plugin mayhem. Some might not play nice due to PHP code security or less-than-stellar server config.

I'd ensure a smooth transition by implementing 301 redirects from all old URLs to their new counterparts. Update internal links and sitemap, and verify the new domain in Google Search Console. Monitor SEO performance using tools like Google Analytics and SEMrush.