Uhm, from my limited knowledge, if you don’t have a Valid SSL Certificate install for your website, you can not use the Full/Strict in CloudFlare. You probably won’t be able to connect to your site.
So either you use Flexible, or you have to obtain a SSL Certificate from the tool provided by InfinityFree in order to use Full/Strict.
And, at least in my case, I manage the CNAME Record, A record… by using CloudFlare, and it works well with InfinityFree.