I would say run a mile any company that wants you to work here on a tourist visa is actively encouraging you to break the law and means they either don’t have enough Thai staff to support a foreigner work permit or they’re just plain dodgy I’m afraid
so you know that it’s going to be down to the immigration officer and they are now really looking at things. So give your best smile lol honestly any agency telling you rubbish like that ignore.
It’s literally down to the officer on the day as if you come in December then you’re looking at approx 160 days this year which means they’re quite within their rights to say you’re not here as a tourist but living here and on the wrong visa so it’s pot luck honestly.
Worst case they say go away and you’re off to Vietnam haha
But the most likely is they’ll tell you this is the last time and you need to have the appropriate visa but it is pot luck