seems to me that if the visa itself is only valid for 5 years like you said earlier then that would mean that you can get back on the last day (after 5 years) and get stamped for 5 more years (so a total of 10 years max). Also meaning that if you need to get in and out of the country after that you’d need a re-entry permit.
it just means the particular officer that particular day may require additional information because they are not satisfied with what you brought. I don't think it's something you should worry about if all your other papers are legit and in order.
I meant renew the extension based on education, my wife used to do that every 3 months at the immigration here in Phuket. (may be the school is closed or something tho)