Lets go over whats gone wrong so far.
I assume the fairground fish where goldfish? Goldfish get big, are very messy, and live decades. They need big tanks, lots of filtration, lots of water changes. If they dont have enough space they get stunted and die young. 10 gallons isnt big enough for 1 goldfish. How many did you have? If they lived a couple of years you actually did a good job.
Soaking wood to remove tannins can take months. Boiling wood is a quicker option. Putting activated carbon in the filter will pull tannins out of the water if you want to put the wood in straight away. I wouldnt worry about any slight pH that might occur when you add driftwood to a tank.
From your own experience you now know that bettas wont tolerate another betta in the same tank. They are called fighting fish for a reason.
What do you know about the nitrogen cycle and how to cycle a tank?
Do any of things you have test water parameters? pH, ammonia, nitrite, nitrate?
Are you seeing any issues in your current set up?
Link to some articles for new fishkeepers that may be useful.