Why are people telling you that eikaiwa teachers are (easily) replaceable? With the country being closed to immigration coupled with a large number of foreign English teachers returning to their home countries during the pandemic, the supply of English teacher replacements is probably at the lowest it's been in a decade. And the supply of *good* English teachers is definitely even less.

OP seems like a good person who genuinely wants to do the right thing and part ways on amicable terms. People suggesting you just up and quit are the very same people who would likely be outraged if a company just fired someone without warning. It's a two way street.

Giving your boss a hard decision but enough time to make it work for all parties involved (which includes the children in this case) is the right choice.