Cron not working after forcing https
The most likely causes for breakage with this change are:

Cron job is pointing to HTTP only

curl by default will not follow 302 redirects, while wget will. If you're using curl, it won't work automatically when pointed to the non-SSL URL (not without -L, anyway). The 302 redirect can make this less obvious, since everything else that points to http "just works".

(Your question says you've addressed that already, but I'm including this for others who face that issue.)

OS certificate chain does not recognise your new SSL cert

Another possibility is that curl or wget is rejecting the SSL certificate provided by your new URL. This can happen if the OS CA chain is outdated or misconfigured; it can also happen if your SSL installation is incomplete (and might not appear to be a problem if your browser includes CA components which your OS lacks).

Debugging further

I would:


Check if your OS has updates available to certificate authority certs (in Debian distros, package is ca-certificates).
Include in this question the output of the cron task you have configured.
Review (and include in this question) the webserver access logs for your cron task accessing CiviCRM. 
Review your site's SSL against a service like Qualys SSLLabs and verify that the SSL configuration is good. (You don't need a perfect score - but this may make some issues visible.)


When doing so, make sure to anonymize the URL and sensitive data like API keys, as you've done in your initial .htaccess snippet.