I "invented" an automatic overflow line for my pool. Conceptually, it's basically an upside-down J-trap under a sink, set to the maximum water height. There's a through-wall return/drain with a grate below the waterline, even with the return. The the pipe loops up to my maximum level, and down into a drain. When the water gets too high it flows over the top of the J trap and into waste. I don't have to do anything and there's nothing mechanical about it.

Carl