Dear Alexandre,
After checking with Suchart (our paralegal), this case is same as Stephanie which You can’t get a visa before travelling. Because the process to issue a letter of approval for the work permit from the Ministry of Labour in Thailand around 2-3 weeks.
So, Suchart suggested that you have to remove all documents from the Embassy. Then you visit to Thailand under visa exemption for Belgium nationality which you will stay in Thailand 30 days.