If you have a single entry Non O then it is “used” on that entry. If you leave on day 75 then the remaining days of stay are null and void. You’ve used the visa.
You could get a re-entry permit to allow you to re-enter for the remaining days allowed if you wanted to. [edit] Didn’t see the bit about the re-entry permit before answering. That only allows you to stay until your original entry stamp. You don’t get another 90 days on entry.
If you have a multiple entry Non O valid for a year (unlikely as very few embassies will issue that these days) then on every entry you’d get 90 days up until the validity date of the visa.
Anonymous participant I recall seeing a post from someone (who used perhaps the Cambodia embassy) where they stated that once you had applied for the visa in that country you were free to travel to any other country except Thailand whilst the application was processing.
Others have posted about successfully applying for a visa having entered Thailand prior to getting accepted and then doing a border bounce to activate it.
You can’t get a visa based on having Thai kids as they are over 20. If you have a Thai wife you can get a visa based on that. Alternatively if you are over 50 you can get a visa based on that.