but all you do is go to Mex/Guat for a day or so and return. Clock starts ticking again.
That won't work always: I once went to Tikal twice in a row and by re-entering Belize the second time, the immigration officer gave me a 2 week visa, stating "I was traveling up and down too much". No discussion.
You have to go to immigration every month. First 6 months it costs 50 BZD after that 100 BZD every time. Theoretically you can do that indefinitely, but the longer you stay without applying for PR or work permit, the more questions they will ask.