Bad news :(
I did a 5 gallon swap of the water yesterday immediately after posting to the board. After I finished working I went to the petstore I purchased the tank from and checked the levels - everything was looking good. Everyone was acting ok. I didn't pull the betta immediately since everything seemed fine/he was acting normal to not shock his system.
All of a sudden last night I notice around 9pm he's hiding down in a corner of a piece of driftwood on the ground. I watched him until about midnight, wouldn't move for anything. As a last-ditch effort I pulled him into a 1-gallon of half existing tank water and half new(treated) water. He's literally dying right now - stopped breathing through gills, takes a breath from the air, starts to tilt, repeat. I don't know if the water change was too much for him, maybe I should have taken him out before. He's had his 1-gallon fully changed all the time with no issues.
I tested the water again last night with my new kit, everything was under control, the pH was HIGH though - 7.0 when they tested at the store, 7.8 when tested at home.
What's most frustrating is he's the reason I upgraded the tank in the first place/I feel like crap for killing my betta.
One thing i did notice - when I removed the dead fish yesterday it had its fins nipped up. Was definitely done after death, the others are unhurt and noone is nipping. I wonder if it wasn't an ammonia issue at all and he was just ill/the betta chewed on him and caught whatever he had.