I used an agent after trying so many over a few years. I'm not a retirement visa but wanted a regular input of 65k over a year. Having a bank of Bangkok account keeps it there and the evidence will be easy. Its something about their systems that doesn't add up, including proper visa advice. In all countries the immigration officer will decide but the process should be more straightforward. Maybe money changing hands has something to do with it
I've used onward travel and it helps but I have never been asked at immigration. I always use EVA Airlines as their luggage allowance is good and direct flights. I suggest get onward travel ready in case the airline asks. But don't pay until its necessary