I haven’t tried this myself so my comment is based entirely from what I’ve read.
Please see http://forum.mikrotik.com/t/web-proxy-on-rb1100ahx4/109339/1 where pukkita suggests that the Mikrotik transparent proxy cannot do HTTPS man-in-the-middle, which makes it ineffective for services such as YouTube.
You might want to consider installing Squid on a machine and using that, although you would need to place a self-signed root certificate in each machine which is to use the proxy so that Squid can intercept: https://turbofuture.com/internet/Intercepting-HTTPS-Traffic-Using-the-Squid-Proxy-in-pfSense