I'm not sure, but I read once that you can include a screenshot of the exchange rate at the time of submission, so if it changes (and it won't change by that much) you can at least show that you had the required amount at the time.
1. A bank statement showing at least 500,000 baht (£11,620). No seasoning is required. It just needs to show a closing balance of at least 500,000 baht. It can be a PNG screenshot of the first page - no need to include pages and pages of transactions. As long as it shows your account details and balance, you're good.
2. Flight itinerary / address in Thailand - Find a flight near the time you want to go and use that flight number e.g. TG911. Find a hotel and use their name, address and phone number. If you're asked for a return date, put something between 3 and 6 months. There was a theory that if you say you're going for less than 3 months, they'll give you a tourist visa instead, but I don't know if that's true, but best to play it safe.
3. A portfolio of your work. You didn't mention what you do, so it's hard to suggest anything here, but when I applied as a video editor, I included screenshots of my personal website as well as screenshots from a video collaboration platform. Both sets of screenshots simply showed thumbnails of the videos I've edited. As I recall, you can only upload a single file here, so I combined all these screenshots into a single PDF. This would also be a good place to include a covering letter at the start of the PDF. Use this to clarify what it is you do for work and also clearly state that all your clients are based outside of Thailand and that you will comply with the rules of the visa and not undertake any work for anyone in Thailand.
4. Proof of address. I used a screenshot of my Thames Water bill.
5. You may also be asked for your most recent tax return. You don't have to upload the whole thing; I think they just want to see that you do genuinely have the business you claim to have. You are allowed to redact any data you don't want to share (they'll tell you all this). You may also need to use a website like ILovePDF to compress it because there's an upload file size limit of 3MB.
On a DTV it's fine. Interviewing someone for a video is well within the bounds of content creation activities allowed on a DTV. It irks me when people say you can't work in Thailand on a DTV. You absolutely CAN work in Thailand on a DTV, you just can't work for a Thai company. The DTV was literally designed for digital nomads to come and work (for themselves or a foreign employer) in Thailand.
Try just using the details of the flight you would take if you were approved. If they ask for proof of purchase as a follow up, buy the ticket then. From what I've read, not all embassies ask for proof that you've actually bought a ticket.