Wrestling would probably give you the most usable fundamentals that would make you a lot better at othe martial arts.
There is even a lot of weapons disarm concepts and restraint and control stuff that come from wrestling. So if you are good at wrestling you will be good at those as well.
Also if your TKD struggles at grindy pressure style striking. You can just wrestle in close and avoid that dynamic. Either bringing the fight to a clinch in your advantage or bringing it to the ground in your advantage. Which will make you win the striking exchange.
And if you do a folk or catch style. You develop a lot of wrestling from the ground to standing up. Which also helps a striking style avoid submissions.
And wrestling is hard. So you will become tough as nails. Which helps as well.
So say for example you wanted to go full ninja.
You could pay big money for a course like this.
Which uses wrestling concepts that are honestly not greatly developed. With self defence priorities applied.
Or you could do really good wrestling for almost no money. And work out your priorities on your own time.