I dunno what you mean about being in the hands of the Visa Sponsor. If you leave the company, your or their decision, the WP will be cancelled and your visa too. You then have 7 or 14 days to leave Thailand. You can do an immediate border bounce no issue. In many countries in the world your visa is tied to your employer -Singapore for example same. It is pretty normal. The job, work permit and visa run together no matter what they are called.
I did not go DTV to B but O to B. Your company lawyers should know the process. At the company I worked at HR did all the processing and mine they had never done before. Every other foreigner hired from outside Thailand or transferred. I had to fly to Singapore to get the B bit company should give you all the paperwork required. Good luck and hope it all goes smoothly.
You need to choose your lifestyle. When I was working in Bangkok I was spending well north of 200k THB a month easily. Not working and not partying or going to "Hot spots" regularly costs are much lower.