I had Non-B visa (with Work Permit) for over 10 years, I moved to marriage Non-O a few years ago. I had the Non-B through my Thai company that I setup and run. In my country (NZ) I was only ever able to get 3 months, but in Australia, they regularly gave me 1 year, multi-entry Non-B.
Extending at immigration was always a pain in the ass, without an agent they just make up any nonsense to not give it to you, because they want the bribe from the agent. For several years, I left the country every 90days, taking short holidays, rather than go through the hassle of extending at immigration in Bangkok. Then each year I would get a new 1 year, multi-entry Non-B back in Australia. When covid hit, I had no choice but to use an agent, which cost me about 47k THB. Bare in mind I run a legitimate, tax paying business, with Thai staff, and hold a work permit.
Then I got married, now I do everything myself, it's much easier, less documents, no ways from them to make stuff up or change requirements on the spot.