Silent rewrite of other TLD on same server via htaccess – PHP Versions

I have a multi-language site with a com TLD with currently the following htaccess rules in place:

RewriteRule ^/?(en|es|de|fr|ja|zh)/$  ?lang=$1&%{QUERY_STRING} [L]

for the home page and

RewriteRule ^/?(en|es|de|fr|ja|zh)/([1-9a-z-]{3,})/$  $2.php?lang=$1&%{QUERY_STRING} [L]

for subpages. This works fine and the user sees something like this:

https://example.com/de/page1/ which is silently translated to
https://example.com/page1.php?lang=de

I now also bought example.de, example.fr, and example.es and they all reside on the same server. Now I would like to change the above structure by showing the visitor

https://example.de/page1/ instead of https://example.com/de/page1/

I tried 2 approaches:

Aproach 1

in the .htaccess file in the example.de public_html folder, I put:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^example.de$ [OR]
RewriteCond %{HTTP_HOST} ^www.example.de$
RewriteRule ^(.*)$ "https://example.com/de/$1" [P,QSA]

However, this results in a 500 error (mod_proxy is enabled in Apache; as I am on a hosted server, I do not have access to the Apache error logs but only to the error_log in the public_html directory, which does not show anything). If I do the rewrite rule with http instead of https it shows something but all the included javascript and style files (*.js and *.css) produce 404 errors so the page is broken.

Approach 2

Instead of doing the proxy rewrite in the example.de public_html, I configured the domain so that its www directory is the public_html directory in example.com. Without anything else, I am seeing example.de in the browser and it shows the contents of public_html in example.com in the (default) English version. I then tried to add some rewrite rules to the htaccess file in example.com to map example.de to correct language:

RewriteCond %{HTTP_HOST} ^example.de$ [OR]
RewriteCond %{HTTP_HOST} ^www.example.de$
RewriteRule ^(.*)$ /de/$1 [L,QSA]

With this, I again get a 500 error. If I use the full URI https://example.com/de/$1 in the rewrite rule, it works but the rewrite is no longer silent of course.

So my questions:
What am I doing wrong in each approach and which one would be preferable from a security and SEO point of view?

Also, and I have not been able to test this as it is not currently working, how would something like this affect my SSL certificates? I.e., which certificate would be applied? The one for example.de (which is shown to the user address bar) or the one for example.com which is actually served to the user?
Thanks.