Betta Fish: Advice for Sick Betta (Velvet Misdiagnosis?)

Hello, I am looking for advice on how to treat my sick betta. Hyperlinks are Imgur links to photos and videos that I took.

I started my first tank about 4 months ago and added a plakat betta, 3 amano shrimp, and 1 nerite snail once it was cycled. I drip acclimated everyone over about an hour. A few days in, I saw my betta making quick jerky movements along plants and decor (flashing, I assume.). Looking at him more closely, I saw that the underside of his chin was a reflective gold color, and his sides looked "dirty". Because of the shiny gold color under his chin which seemed to be spreading, I assumed he probably had velvet disease.

At this point, here are the tank details:

• 10 gallons
• 50W Heater set to 78 °F and filter included
• API test kit used 1-3 times every week: pH – 8.1, ammonia – 0.0 ppm, nitrite – 0.0 ppm, nitrate – ~15 ppm
• Weekly 25% water changes with dechlorinated and temperature matched tap water. The tap water in my area is very hard and I got the following test results from it: pH – 8.1, ammonia – 0.0 ppm, nitrite – 0.0 ppm, nitrate – ~15 ppm
• I was feeding 3-4 Northfin Betta Bit pellets twice a day
• Tank is decorated with a dragon rock, driftwood, betta hammock, betta log, and planted with frog bit, anubias, bacopa caroliniana, alternanthera reineckii, hygrophila corymbosa, and monte carlo. There was a plastic cave decoration that I threw out after hospitalizing the betta; it seemed to be getting dirty with snail poop and maybe algae (not too sure).
• Picture of tank back then

I created a bare-bottom hospital tank and dosed it to 0.3% salt and 0.25 ppm Seachem Cupramine. I set the temperature to 78 °F and increased it by one degree per day until it was 83 °F. I drip acclimated my betta before putting him in here. Here are the hospital tank details:

• 2.75 gallons
• 25W heater and sponge filter included (mostly for aeration)
• Tested daily for the first few weeks and then weekly. pH – 8.1, ammonia – 0.0 ppm to a hint of green occasionally, less than 0.25 ppm, nitrite – 0.0 ppm to a tinge of purple occasionally, less than 0.25 ppm, nitrate – ~20 – 30 ppm
• 25% water change every other day using dechlorinated temperature matched water, 0.3% salt, and the desired cupramine (copper) concentration using Seachem's dosing instructions.
• No tank mates
• Continued 3-4 Northfin Betta Bit pellets twice a day
• Tank decorated with silk plants, betta log, and betta hammock. From beginning to now, it was covered in towels all day every day except during feeding to block light.
• Picture of hospital tank

He tolerated the treatment well and remained curious and hungry, but I didn't see a change in his appearance for about a week and a half. At that point I increased the cupramine concentration to 0.3 ppm. After 2 weeks, there slight improvement but progress seemed too slow so I bumped up the cupramine concentration again to 0.4 ppm. I tried using the Seachem copper test to verify the hospital tank copper concentration; I had trouble reading the tests precisely but they seemed close enough. There was a bigger improvement this time; after 2 weeks maybe around 20% of the gold coloration had receded, and the rest was now a dull color instead of shiny. For the next 2 weeks, however, he continued to remain the same; pictures of betta at this point.

Thinking that the velvet might be dead and I was just looking at damaged tissue, I decided to move him back to the main tank to see if he would heal. I was also worried that an extended copper treatment would be harmful. I began doing 35% water changes every day for five days with no more salt and no more cupramine. I added almond leaf tannins to the main tank and drip acclimated the betta back into the main tank the day after the 5th no-med water change.

Some changes to the main tank while the betta was hospitalized:

• I add pressurized CO2 with a diffuser one hour before lights until one hour before lights off, so pH was now testing between 7.4 and 7.6, depending on time of day.
• Added water wisteria and salvinia. All of the plants really took off so now my nitrates stay around 5-10 ppm, but they would be zero without fertilizer
• Now I do 25% water changes every other week. Ammonia and nitrite continue to always test 0.0 ppm.
• After not seeing them for over a week, I thought 2 of my 3 shrimp had died. I added 3 more and soon after all 6 shrimp became active enough that I managed to see all 6 at the same time.
• Picture of tank now.

4 days after putting him back into the main tank, I see that I made a mistake because he's clearly getting worse. The gold areas are shiny again and seem to be spreading. One of his sides got really bad, especially at the base of his tail. The coloration under his chin is shiny gold but his sides are more of a brown color. Here's an album of pictures and videos I took today; he's usually a really sleepy guy at night, but he seemed even less responsive than usual tonight.

The copper/cupramine seemed to keep the disease suppressed but was unable to kill it as quickly as I expected, so I want to try something different. I am considering re-hospitalizing him after I get some sleep and treating him with Rid Ich Plus. I've prepped the hospital tank tonight with a 100% no-med dechlorinated water change and will visit my LFS for the new medicine once it opens.

But I'm worried that I misdiagnosed him. I looked up more pictures of freshwater velvet and realized that the gold coloration covered a solid area, rather than many separate specs like I've seen in pictures of velvet online and in books. Please let me know if you think it might be something else, and/or if you have any recommended treatments. Thank you so much.