Betta Fish: Sick betta, and IDK what to do.

Hello all,
I haven't posted here in a while. I own a betta fish named Columbus, and he should be about 2 years old. I've had him for almost 2 years too, so. He lives in a 6 gallon fishtank.
PH: 6.8-7.2 about.
KH: 10-15°
NO3: <10
NO2: <1
Last time he ate was 2 days ago. He's been slow, but not entirely lethargic. Comes up for food, doesn't show difficulty swimming. No bladder issues. Fins look okay, no tears, nipping, etc.

But he has a tumor(?) by his eye, and a diamond eye. He also has lymphocystis – he's had it since I got him, but these past two months have been the worst. He's endured the disease for as long as I've had him, and every time a cyst popped and deflated, I treated his tank so he wouldn't get an infection.

He has many shrimp in his tank that he's friends with. They don't stress him out, he isn't hostile to them. I've seen him curiously observe them many times. I catch some every couple weeks and put them into another fishtank, where they get eaten, usually.

I'm not sure what to do. I'll see if he wants food later today.

The tank looked much different a day ago. The glass was covered in hardened algae that the shrimps or snails didn't wanna eat, and I finally got the tools to scrape it off. I removed him from the fishtank and put him in a jar for a single hour as I was cleaning (you can see it's not perfect, but we're getting there). Also replanted his plants, and removed a big centerpiece wood that's been just taking up space for everyone. His favorite hiding spot is the cave in the center. He also sleeps there.

Is it the change that stressed him out? I don't know, because he was a little odd a few days before this.

Please let me know what to do. Lmk if I'm missing any details you need. Thank you in advance.