At this point, I'm thinking I have to completely disassemble it.

Yeah, I did that but when I start trying to loosen this nut here, the actual fixture inside starts to spin.

Oh, it worked. I just had to grab just the nut?? I'm not entirely sure what I did differently the second time, but it came off in the fixture inside I did not spin.

I think that last nut on the bottom is supposed to unscrew. Probably corroded.

Is that a spacer separate from the nut? If so, you may have to crimp it with some vice grips to hold the rod still while you bust it loose.

If not, you may be forced to take it all apart to get it all loose.

I think you are on the right track, but the fixture isn't working as intended.

Ya my parents have some like that. Always a pain in the ass as everything corrodes and strips or breaks free. I just replaced mine with some that you don't have to disassemble anything. Too humid on the coast.