I am a bit doubtful that recovering server resources is going to make enough impact to your business to make a difference. Removing 25% of your worst performing pages isn't going to remove 25% of your traffic. I doubt that you will be able to downsize your servers as a result. Even if you could, servers are cheap. A small amount of extra resources spent on servers rarely makes a big difference to a business.
If these pages are costing you in human capital, you probably have a better case for removing them. Pages that require editor, moderator, or developer time end up being far more expensive than the server resources they consume.
When evaluating pages to remove without hurting your SEO, you should consider additional metrics such as how many external inbound links a page has. Pages with lots of links should be kept, regardless of how badly they convert. They are helping your SEO far more than pages without links.
Removing low quality pages from your site can actually help your SEO. See Delete your pages and rank higher in search - Index bloat and technical optimization 2019. Deleting your pages that don't convert and don't have links is unlikely to hurt the rest of your site, and could even boost its SEO.
As far as the mechanics of removing pages:
- Remove all internal links to the pages that are getting deleted. You don't want users or search engine spiders to browse to error pages from your good pages.
- Use appropriate HTTP status codes:
- Use "301 Permanent" or "303 See Other" redirects when there is a page that is being kept that is an appropriate replacement
- Use "410 Gone" status for pages removed without a replacement. "404 Not Found" is mostly the same from an SEO standpoint, but search engines take 24 hours longer to de-index it after the crawler finds it.