Any chance of trying this again? I'm not sure what happened with the previous attempt with a Let's Encrypt certificate, but was there a reason why it couldn't be used?
The version of SMF we're currently running doesn't support SSL. As such, if I turn it on a fair amount of forums functionality (images at very least) will break. I am planning to do an upgrade when I get the time, but given this is a production environment it's not something I want to do in a rush.
The next release of Chrome is the one that is meant to flag anything HTTP as unsafe, so people might start to hit problems. If you wanted any assistance at any point, just let me know.