Do Thais or Cambodians even need a visa for China?

What kind of visa it? Never heard of an 81 day visa or 21 day renewals.

I have been going there since the late 70's and never been asked anything or stopped either.

Apply for your visa first. As long as you meet the criteria you will not be refused a visa. Enough money, a return/onward ticket there should be no problem. Saying that it appears to depend on which Embassy/Consulate you are applying to. There are a 20+ odd Countries that have to apply at the Thai Embassy/Consulate of the Country issuing their passport, Tunisia is not one of those Countries.

Why don't you apply to the Embassy/Consulate for a retirement visa yourself and do away with the agents. If you meet the criteria there is no problem normally..

Retirement

Good luck with that . You are better off doing it yourself. Check the ads in Pattaya mail etc.

The airlines I use always check the dates and my visa. Never get asked when I have a re-entry permit. Thai Immigration have never asked to see a return or onward ticket..

I either have a return ticket or a full fare onward ticket that is fully refundable.

There are plenty of Thai Insurance Companies and they are cheaper than any others. I am 77 and just got 3 months for CAD 300. All based on age of course. All of the N American Companies I looked at were way above that.