301 redirection

We are going to delete old HTML website & replace it with a new redesigned WordPress site. We already have http to https 301 redirection in .htaccess file. Is this redirection is enough to maintain SEO? Or are we need to add extra page level 301 redirection for its equivalent wordpress pages

It's about content location, more precisely the URLs of pages. You have to set up 301 redirects from the old URLs to the new ones. If the URLs and their structure don't change, you don't have to do anything specific then.

Either way with a complete site redesign things will change and you have to expect at least some drop in your rankings temporarily.

if you use cloudflare, create a new redirect, purge cache and you're done. easiest way

https://moz.com/blog/website-migration-guide
check this out