You definitely need to translate everything to English - most Thai embassy sites mentioned this. (You could probably do Thai translation but I think English is safer in order to ensure a likely more accurate translation).
I'm waiting to collect my passport on Tuesday so I'm still not a 'successful case' yet, but since my application was a walk-in and they accepted the documents and my money it's likely that I will be. When I was lodging my application, they sent a guy away for not having a 'letter' from the company he worked for. He came back a little later and said that he owned the company so why should he need a letter? They finally accepted his application without the letter but it caused issues. I didn't have any issue like this because I wrote a letter explaining that I'm a self-employed freelancer and that my business structure is 'sole trader' as it is called in Australia. I provided my Australian Business Number in the letter and a supporting document. I also explained how I receive money from my clients (in my case from Fiverr to Payoneer and then my bank account) and showed evidence of the payment trail in my accompanying documents. What came across to me from their questions is that the most important thing is to provide documents that allow them to understand your business and how it works and to prove that you actually get paid. I think their main concern is that (on top of the required 500k baht) you actually earn an income so that you won't be likely to work illegally in the country. Actual examples of work might be good but I didn't provide that. I provided reviews from Fiverr (my printed Fiverr profile) and one contract with a private client and an invoice to that client specifying the service I was offering. The officer glanced at these things but spent the most time reading my letter and then asking questions about my payments and bank accounts. I hope this helps.
Thank you for posting this. I have the exact same situation as you - ED extension plus re-entry permit. I've got my appointment at the Vientiane embassy tomorrow and I've been worried that this might cause issues, but it sounds like it shouldn't be a problem. Will report in when I know the outcome of my application to confirm.