When you are asked for additional docs the approval time is reset to back to 0 days. UK is fairly quick with approval so you are vry likely to hear something soon. If you are out of the country when approved and they find out your DTV can be cancelled or rescinded on arrival in Thailand.
It can take 10 business days to get approved. You can also be asked for an interview so it is best to allow for longer than 6 days. As a business owner you just write a letter to yourself giving permission to work remotely in Thailand. Your tax return should be fine they just use it to verify you working legally.
They ask basic questions on your application. You will be asked to show your bank balance on your phone. Soon after you will likely receive a message of approval. Bring the docs you submitted.
July - Aug is just 2 months. You need to provide 3 e.g July, Aug, and Sept. Even on SP, embassies want evidence of income. If you are unemployed, submit a letter again stating you are unemployed. You can
include additional savings to show you can support yourself in Thailand with no job income. They are looking to see how you are going to support yourself with no income.