Believe there is some confusion here. If you’re changing the .htaccess and getting an 500 Internal Server Error, you need to check what you have there. As mentioned, you will want to check if you’re setting the PHP version there, if you’re setting this there then you’ll need to carry that over. Unfortunately, as you have so many custom items in it, this would need to be managed by yourself as you are responsible for server items, such as this.
If you disable all custom rewrites and redirects and get the site running on the correct PHP version, we can certainly take a further look.