Keep comments but turn off commenting

What you’re describing is theme specific. The default themes don’t do this, and instead simply state that comments are closed:

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If comments are turned off, they change the comment form. But your theme appears to change the entire comment section instead of just the form.

You’re going to have to either modify your theme or ask your theme vendor to fix this. Take a look at comments.php, it’s probably in there, but it may be implemented differently in your theme. To find out how you will need to look at the PHP code. Take a look at how twentytwenty and twentynineteen do it for what it should look like.

Also, make sure you use a child theme to make the modification or it’ll be undone when you update the theme