Ok first of all, holy crap, I can't believe this actually hit the front page, definitely wasn't expecting that. Thanks again for all the input, I couldn't have asked for more.
Ultimately, I'm still not 100% sure, but after going through all the comments a couple of times, I think I've finally made up my mind. For the next month / couple of months, I'm going to slowly get back into freelancing while giving PHP a serious go. Although PHP may not be my first choice, I realise now this is actually the right one since WP development is going to be my main source of income for the next 6-12 months, so I need to get really good at it. Aside from that, I plan on sharpening my front-end skills and trying out a couple of other stuff as well such as SASS and Git.
After I feel I'm comfortable enough with PHP, I plan on trying out the Laravel framework, and see if I actually like it as much as Rails. Either way, I'm definitely going to learn Ruby as well ( I've already finished "Learn to Program" by Chris Pine some time ago ), and eventually spend less time on WP in order to finally master Rails. In the end, I'm hoping to give up WP entirely and work only on RoR projects ( either my own or by freelancing ).
I know a couple of you guys were suggesting that I should focus on things which are not related to web development at all, and I see your point, but honestly I simply can't get excited about that stuff at all. Maybe later on I'm going to feel different about it, but for now I'm focusing solely on web development.
Also, regarding SQL, that's something I may've left out in my original post, but fortunately I am pretty familiar with MySQL, I even studied it in school.