You'll never lose a fight again

Wow. That's rather impressive.

Sport and American TKD yes. As used by more militant and modern styles (like Korean armed forces) the kicks are used to bridge and control distance and hand work is key. But most TKD incorporates kicks meant to dismount horses riders and anticipates "peasant weapons" and other relics. It's also not a simple martial art to be able to use. Many kicks require frequent stretching and practice to be able to perform correctly, you need space (in a largely enclosed modern world), and it takes many years to become an effective fighter for most. The wisdom is never kick someone with a knife, and knives are the most likely weapon you'll encounter in the real world. All in all I'm with you. Lot sense of good reasons to take TKD, and even sport or American give most of the benefits of the more combat oriented styles. It's a fine style but I don't reccomend it as one to rely on in self defense unless your serious about practice and dedicated with a great teacher.

Or carry a gun.

I’ve had my ass kicked before. You get over it. I’d rather that than shoot someone.

Guns are... mixed. Closer than 7' of it's not drawn or ready to go you're usually fucked unless you get the drop (but that's not self defense..) which is the next problem. Depending on where you live carrying or using a gun even in defense may end you up in jail. Guns escalate things too. If I'm out to cause trouble the first guy that goes down and HARD is the one with a gun. He's target #1. Close in knife beats gun, and with a gun in play "run" stops being an option for them, it's take you out or get shot. I like to take people to shoot and have them sprint a big lap with a holstered pistol, I tell them when they're done run straight to the line, unholster as fast as possible, and fire 4 shots asap. They're usually shakey and fumbly. Most don't even hit the target and most unload the magazine because they can't count. Shooting at a person close up under stress isn't like plugging a deer or paper. You're likely pumping adrenaline and scared. Bonus points: they are faster and take it.

A good single martial art would be one of the Muay variants. Next would be a ground grappling art, then a standup grappling art.

I overall agree. Much like boxing Muay varriants can be complex and skilled arts- or they can be pretty effective and brutal with light training and an ok teacher. They also feed into and off of common sports and exercise hobbies like weights or jogging, cross fit, etc. Elbow and knee strikes are good for tight spaces and protecting your hands and feet of you don't have great form. I hate grappling but most real (unarmed) fights tend to involve it in some way, so I'd say at least spar a grappler from time to time. But for most people grappling arts are also good for defense and knowing how to avoid or escape common grapples. They also don't take a lot of training to use well enough to function, but you can spend a life trying to master them if you want.