This all depends on what you want partner XD! You have two options I see from what you have given me.

1. Expand on your Tkd by reinforcing it with kick boxing or MT. Both will increase your Kick strength and body control significantly, as well as add more advanced blocks, strikes, Bobs and weves, and introduce you to something completely new..... THE CLINTCH!!! XD. This opens up so much potential for your striking game, so you were smart to use a form of karate as a fundamental.

2. Mix things up, and acquire a "ground game". This will give you much more versatility in your arsenal and mindset. Also this will unlock your ability to think while your back is to the wall.

The choice is yours my fellow martial artist! XD!!!! Personally I'd go with the striker route because it's what I did when I was at your "point of crossroads" haha XD! Good luck partner! :3

Jiu Jitsu is not very effective in a the street

(unless ofcourse you are a Gracie)

But it would complement your taekwondo training more as you would be learning about the other side of fighting

Muay Thai or kickboxing are basically the same thing give or take a few and would be easier for you to pick up as you already have a lot of experience in that field

So if you are trying to learn and become more well rounded I would say Jiu Jitsu

If you want to sound dangerous on the street ummm Muay Thai

And 9th degree? seriously? all the best for that

Maybe your best bet is to go into muay Thai and JJ because muay Thai from what I learned is defend then immediately strike paired with JJ and tkd is deadly and no one will want to be with you in a dark alley unless they are your friends or family

Brazilian jiu jitsu goes great with tae kwon do. If someone catches your high kick you will be forced to fight from your back.

I currently have my Black Belt in Tae Kwon Do, and I'm still gonna train to get my 9th degree eventually.

The 3 martial arts I have chosen are...

Brazilian Jiu Jitsu

Kickboxing

And Muay Thai

Which one if these martial arts would make me an overall better fighter, strengthening my standup with Tae Kwon Do, and Kickboxing or Muay Thai. If so, which one is better, Kickboxing or Muay Thai.

Or mixing both standup and ground with Tae Kwon Do and Jiu Jitsu.

Just wondering which would suit me best, making me a well rounded fighter with good experience.

Which would sound scarier in a street situation,

I'm a blackbelt in TKD and I'm in Jiu Jitsu

Or I'm a blackbelt in TKD and I'm in Kickboxing/Muay Thai.