因WordPress 外掛Elementor 存檔跳 500 Internal Server Error,檢查了Nginx的Error Log,連帶發現
[warn] a client request body is buffered to a temporary file /var/lib/nginx/tmp/client_body/000000000x
參考:Nginx buffered to a temporary file 问题解决
修改nginx設定後即解決,如下。

##原有設定##
location ~* \.php$ {
   fastcgi_index   index.php;
   fastcgi_pass    127.0.0.1:9000;
   include         fastcgi_params;
   fastcgi_param   SCRIPT_FILENAME    $document_root$fastcgi_script_name;
   fastcgi_param   SCRIPT_NAME        $fastcgi_script_name;
##加入下方設定##
   fastcgi_max_temp_file_size 0;
   fastcgi_buffer_size 4K;
   fastcgi_buffers 32 8k;
   client_max_body_size 500m;  ##這筆是用來調整上傳限制
   client_body_buffer_size 1024k;

此問題與memory_limit可能要一起調整,才能完整解決 500 Internal Server Error。
因一開始只針對Allowed memory size of 268435456 bytes exhausted (tried to allocate 331776 bytes) 來處理時,
提高限制為300MB一樣會跳錯,最後兩個都改完就沒跳錯了。