BJJ seems like a really good option for self defense. As samw34099 said, most fights usually end up on the ground so being able to defend yourself in that position will be extremely useful as a lot of people are unfamiliar with some of the principles of grappling. The only potential downside is that BJJ is mainly a 1v1 system, that's not to say that skills such as muay thai and boxing will allow you to defend yourself against multiple people but BJJ can leave you more vulnerable as you're likely to be on the floor. A lot of BJJ gyms actually teach striking martial arts too so maybe you could give that a go if interested but even practicing BJJ gives you better self defence than the vast majority of people. Read quite often on MMA forums that the best defense is unironically to practice your running/sprinting, even trained MMA fighters have ended up in serious danger in various situations so I think being able to run effectively is invaluable