just be aware there are companies that are not licensed for insurance in Thailand or the country they claim to be from. Their premiums are cheap but when they refuse to honour claims you have no recourse.
the only problem there is if you have any pre-existing conditions you will not be fully covered. Also as you get older ie over 70 5000 a month will not be enough to cover your big toe without significant deductions
I guess that by public healthcare you mean Govt hospitals. As a foreigner they are not free but are considerably cheaper than Private Hospitals. Some Govt Hospitals are very good but generally are very busy so be prepared for long waits for certain treatments.