The OP actually knows where their application was weak. Provided "uncredible employment documents" 🫠. This remark was on their soon-to-be-canceled visa too. OP first sent documents which were not adequate. After a request for further documents, OP added documents which contained the disclaimer: "this document has no legal value" and no other document/explanation. The document was actually OK, but this mention is standard and it definitely made a bad impression. OP should have sent another one, or at least provide clarification.
Typical. One is supposed to deposit funds in a Thai bank for non-imm extensions, yet they make it difficult for you to open a Thai bank account. One is supposed to file returns, yet the revenue service will give you various reasons for not issuing a TIN.