2000 4Runner Starter Issues

Hey all, I've been a long time lurker but have never actually started my own thread. I'm in desperate need of help, so the time has finally arrived.
My car's been making an ugly grinding noise the past week. With visual inspections, I found nothing wrong. But last Friday it cranked and immediately died. I popped the hood and the negative terminal and connection were fusing together so I quickly popped off both positive and negative connections. I assumed it was something wrong with the battery, so I replaced it. Upon putting in the new battery, it sizzled both terminals/posts again so I quickly pried them off.
I suspected the starter, so I took that out and it was very, verk broken. In fact, the starter pinion was lodged in the flex plate (my 4Runner's an automatic). I cranked the engine pulley a turn which freed up the pinion.
I carried on and put in a new Denso starter (#2800166) yesterday. The 4Runner started up, made the same ugly grinding noise as before the swap, and the starter promptly started smoking.
I'm no mechanic-- this is my first car and it has slowly been turning me into a "car guy", but I have very little experience, so any and all help is appreciated.
I assume it's an electrical issue but I'm lost. I have found no ground issues or wiring issues in general.
Thanks for taking the time to read and for any help!

Edit-- if helpful at all: 4Runner is a 2000 SR5 automatic that just hit 202k miles