I’d go with closed cycle Stirling engines. Even better efficiency than open water boiling. The general rtg use cases care less about efficiency than long term

I’d go with closed cycle Stirling engines. Even better efficiency than open water boiling. The general rtg use cases care less about efficiency than long term reliability though so that’s why they use thermocouples. But it would be reasonably trivial to use a Stirling engine instead (unless you’re going to extreme environments where mechanical parts no longer work well)