How can I obtain a Non-O visa to open a bank account and deposit 800,000 baht in Thailand?

Judging by your profile you are from the UK so I'm going to assume you have a UK passport. If you can show income of 65k baht for the last 3 months or 800k baht in your bank account you can apply for the 90 day Non-O visa before you leave the UK using funds in your UK bank account. Once you arrive in Thailand you can open a Thai bank account and then place 800k baht in that in the first few weeks you are here, once that's been in place for 2 calendar months you can then apply for a 12 month extension of stay

It's only 40,000

Sounds like things will fall into place once you have the equivalent of 800K in your home bank

Once you have that

When you are ready to come - Apply for a Non-O 90 day Visa from the Thai Embassy in your home country about 30 days before departure.

When applying online you will need to show the required funds in any bank account , some personal details & Flight details & proof of first nights accommodation .

Once approved & arrive in Thailand you have 90 days to organise for the 12 month extension

Once you arrive You have 90 days , firstly set up a Thai bank account & deposit 800K Baht for at least 60 days to satisfy the extension