Wow, I am sorry if you had such terrible experiences with the dens of corruption. I respect your choice to use an agent (which however, in some cases though not yours, is a way of continuing the corruption). But I have to say that your dim view of Immigration offices is at odds with mine.
I deal with the CM IO since a dozen years, since a few years for retirement extensions based on fixed deposit. I have to say that -knock on wood- it's always gone rather smoothly, if I had to wait it's usually because of an agent jumping the line with several passports, or because of the marriage extension guys with an expecially talkative wife. Since two years I use the online booking system and I am in and out faster than I can check the news on my phone. Without genuflecting. A wai is a kind gesture and I do it without feeling like a clown.
So be happy with your choice, just as I can be happy without an agent.