Ok then you won’t need anything from Belgium to get a local extension. You will need a Thai bank account and 800k deposited for at least 2 months prior to applying. There will be other forms to fill and things to get from your bank etc, but these vary from office to office which is why I suggest you contact them for their list.
Assuming your passport is from one of the 93 that allow visa exempt entry you won’t need a visa for up to 90 days stay. You’ll get 60 days on arrival and can extend that once for a further 30 days at a local immigration office.
Your airline may have a hang up if you don’t have a ticket out of Thailand within the initial 60 days allowed regardless that you could get an extension. Some don’t care, some insist.
It doesn’t have to be a return ticket, just a ticket out of Thailand.
Google “onward ticket” and you’ll find loads of companies that will sell you a ticket for approx 14 days. It’s a valid reservation on the airlines system but the ticket won’t be issued. Cost is usually about $12.
Many that can afford to do so indeed have separate accounts. One for immigration where they leave the amount untouched and use another account for spending. No issue with that other than the pretty much lack of interest on the 800k. Lucky if you get 1%