Breaking news: I decided to retry bonding and I am cautiously optimistic that they are going to make it!
So the backstory:
Bandit's cagemate and twin brother, Bolt, was put to sleep on 12/2/23 for cancer near his airway. I got Bunker. They lived side by side for a week, then I tried introducing in a neutral space. Bunker wiped the floor with Bandit, I separated, Bandit acted like a PTSD victim for a few days, and eventually everything normalized with them living side by side. In retrospect, I think Bunker was withdrawing from marijuana, because he was REALLY chill the first 3 days, and his poops smelled like weed. Bunker was a rescue that I got from a family that found him in a park.
BUT THEN Bandit got increasingly agitated over the past month and kept trying to get over onto Bunker's side, always chewing on the bars or trying to dig underneath the grate. So I put down a clear plastic divide over the grate. He now would try to bite the plastic, so I put down a cardboard divide over it. He just would not stop trying to rip the cardboard off. Even his favorite treats would only distract him for a few minutes then he'd get right back to trying to break down the wall. He was a piggy obsessed. So finally after a month of that I decided to take down one of the dividing grates. (I know, I did not follow protocol, but I went with my gut on what the piggy wanted). The first hour they'd cycle through humping/chasing for a couple of minutes, then stopping/getting distracted by exploring the side of the cage where the other piggy has been living, then going back to it. Then the cycles would get longer and longer apart, then finally they started to plop down next to each other for naps!

Here is Bandit (left) and Bunker (right), flipped from their normal sides. I guess they think the hay really is greener on the other side.

Here is Bandit (left) and Bunker (right, under the bendy bridge) taking a nap next to each other.
I'm not positive because they both hump each other, but I think Bandit is top piggy now because he's rumblestrutting more, and humps Bunker more than the other way around.
They've been together about 5 hours now. Last time they lasted maybe 15 minutes before I felt the situation was getting too aggressive and separated.
Fingers crossed that they work out!