Any such aliens will have to be simple enough, or clever enough, to hide from the Thargoids.
Perhaps something like the Yilane of Harry Harrison's West of Eden series, or the Yuuzahn Vong from the now-retconned Star Wars sequel novels - critters who use an entirely biotechnological-derived civilization, which would evade the sensors of the Thargoids who are looking for technological signs that a new sentient species has arisen.
The other alternative is a "First Ones" solution, an advanced race who had fought the Thargoids to a standstill in the remote past, but gave up interstellar colonization in the face of repeated Thargoid attacks and retreated back to their homeworld, maintaining the capability of defending their home from the Thargoids - and anyone else. The Thargoids would probably be tolerant of such races continued existence, so long as they never again ventured forth from their fortress system.
Perhaps something like the Yilane of Harry Harrison's West of Eden series, or the Yuuzahn Vong from the now-retconned Star Wars sequel novels - critters who use an entirely biotechnological-derived civilization, which would evade the sensors of the Thargoids who are looking for technological signs that a new sentient species has arisen.
The other alternative is a "First Ones" solution, an advanced race who had fought the Thargoids to a standstill in the remote past, but gave up interstellar colonization in the face of repeated Thargoid attacks and retreated back to their homeworld, maintaining the capability of defending their home from the Thargoids - and anyone else. The Thargoids would probably be tolerant of such races continued existence, so long as they never again ventured forth from their fortress system.