how to cache password protected pages, one set for all users

I have a small photo blog for friends and family only so the pages are password protected. Regrettably the server’s php rendering can be slow sometimes so I want a server caching solution. Regrettably it seems password protecting raises obstacles for this.

Even if I activate “Enable caching for all visitors” each visitor is treated individually so the first time they visit no cached files will be waiting for them, and when they visit again three months later (typically because I have uploaded a new gallery and informed them) they likely will not have the cookie still, or may even use a different browser. So they will effectively never be served cached content, other then when re-visiting a page they viewed very recently.

Is there a caching solution for this scenario? The pages barely ever change, so I could have TTL for a year and take it upon myself to regenerate the cache whenever pages change. But since they clear cookies or use different browsers they will be considered as new visitors and a cache will be built for them every time they visit forcing them to get long load times.