As this SBC will be used for educational use and not for any industrial/professional application. In my opinion a RC reset is good enough for SBC.

From various posts of members it appears that most easily available and very widely used supervisor is xxx1232. But it has a hardware WDT and no way to disable it you need to keep it feeding through ALE that's a sort of disabling it.

If you want a simpler smaller supervisor with Manual Reset and without WDT the best thing I have seen/used is MAX812 it's very tiny [ space saving only 4pins in SOT143 ] and is very capable. But it is not so easily available in most of the parts of globe.

Regards,
Prahlad Purohit