I have almost exactly the same problem as my landlord never completes my TM30. I used and agent and it cost 1500THB, money well spent as it meant I didn’t have to go to immigration either.
Can honestly say I have never heard of anyone being forced into paying immigration a bribe. Obviously if you use an agent there’s a high likelihood that they are paying a bribe on your behalf to expedite your processing time, but if you have not made an error I can’t see how the IO can extort you.
honestly I would definitely recommend you check what the maximum pay out is. If you end up in hospital for a week the bill could easily be 1mTHB. I am paying 40kTHB for 5mTHB cover
I have insurance and made a claim in the first year and it took about three months for them to pay out, as they wanted to check with every single hospital that I had ever visited in Thailand and confirm that I had not received treatment previously. I had not, I have only had medicals for my work, but I had to give permission for each hospital to release my information, even though I had already signed a consent form with my insurance company.
When you take out the insurance you complete a medical history form, and they will then decide whether to exclude the cover depending on the information that you give.