How do I get rid of stubborn overgrown weeds the size of trees?

If you have a co-op or tractor supply, some type of store along those lines, visit them. Take pictures of the weeds and they can advise you what you use and the proper strength. We use Cornerstone which is an agricultural weed killer. It's the same thing farmers spray the fields with when raising round-up resistant crops.

You will have to dig it up and keep digging every time it tries to come back. Try these tips for smaller weeds in the area: https://www.craftylittlegnome.com/permanently-kill-grass-weeds/

Salt will kill anything it touches. But be careful because salt will kill anything it touches. Good luck.

I use clorox, but you cannot plant anything there for a long time and you must not use if you have young children or pets.

Cut ot down as far as you can, dig out the roots. Lots of hard work, but worth it. I've been doing a LOT of this for months now in my overgrown yard