Have you tried the ranked mode? There should be SBMM there. It's still in the testing phase though. I am an average player who also plays solo like you, but I have a lot of fun. There is always someone dominating the game in every online FPS game, and if you are not playing very well, SBMM might not be able to prevent this.
I am fine with doing bad for a few games, but with Xdefiant you can never really have a good game since you will always be playing against top tier players, when i win a game its never because of me its because some guy on my team went 60 and 10 and when I lose its the same reason. Honestly it just makes you feel like you are there so somebody else can have a good time. Screw that, I have been more than happy with aggressive SBMM and have not heard one argument against it that stands up to any scrutiny for somebody who has a life out side of playing games 8 hours a day.
edit: I missed your ranked comment. Ranked modes are not fun, they usually have longer play modes , people are assholes and expect specialization, when you lose you get demoted right in fort of your face which is demoralizing and the team shits on you if you played bad. That is not what i want out of SBMM, I am just playing to have fun but I want to feel like I contributed on the scoreboard in a meaningful way.