Nginx PageSpeed Benefits of ngx_pagespeed

at scale Cloudflare Pro would be better as there's more server resources + CDN caching to nearest visitors' locations usually means better page load times but depends on where the majority of your visitor audience is too.

though it doesn't hurt to have ngx_pagespeed on backend and cloudflare on front end either as all cloudflare is doing is CDN caching those optimised assets to serve from their nearest datacenter to visitors. The end goal is to always try to minimise the assets and data set size of transfers from point A to point B. So from your server to Cloudflare and from Cloudflare to visitors or from your server to your visitors.