A toggle/function to cache/download a webpage so there is an "offline" copy/version

Question

I'm trying to get a webpage working so that a page that constantly updates throughout the day can continue to function like normal and if possible cache correctly/better.

Currently I'm using the following htaccess file:

<FilesMatch "\.(html|htm|js|css|php)>
    FileETag None
    Header unset ETag
    Header set Cache-Control "max-age=0, no-cache, no-store, must-revalidate"
    Header set Pragma "no-cache"
    Header set Expires "Wed, 11 Jan 1984 05:00:00 GMT"
</FilesMatch>

Which basically is making everything re-download every time they refresh, it would be good if there was/is a way to only make it re-download if something has changed (I have no idea how to do this, could it be done by checking one specific file or something?)

I am also trying to get a way to have a "Go Offline" or "Cache Website" (Better if it is automatic) so that it caches/downloads a copy of the entire webpage + some other pages of my choice (if possible) which would mean if I was to load the page on a mobile phone and then disable wifi/data and refresh the page it would still be accessible.

I've tried using the manifest files (.appcache and .manifest) but it just seems to cache the entire page regardless and when I make a change to the webpage it doesn't update even if the data/wifi is enabled, I have to clear the cache before it works...

Any ideas?