Can I leave Thailand temporarily and return to apply for a 12-month family visa while waiting for custody papers?

Another option that you have

@Jannik *****

is

Rather than bounce out and back to get a 30 day free entry and then apply for the Non-O inside the country why not just go to the thai consulate in Vientiane or Savannakhet Lao and apply for a 90 day Non-O visa based on being the parent of a thai child.

You can get that for 2000baht with just your name on the birth certificate, and copies of the thai mother's i/d & house book listing..

That way you when you come back you'll get a 90 day entry stamp and can go for the year extension from that once you get the parental rights decree sorted out

That would cut a few steps out of the process and as you said you have to exit/re-enter anyway (y)

Just a thought

There is no 12 month family visa. You are talking about going from the 90 day Non-O visa to a year extension of stay based on supporting thai children.

You should be able to bounce out and back entering on a free 30 day entry.

Then apply first for the 90 day Non-O visa for 2000baht and 2 months after that apply for the year extension for 1900baht

You have to start all over from zero when you bounce out and back because you need a Non-O visa to get the year extension and you lose that Non-O you're on when you stamp out/back in on the free entry

You would need to re-enter with a visa exempt, then with more than 15 days remaining apply for a new Non-O at immigration, and then apply for the 1 year extension