You could try (if you only want to add .wav as extension)
Format string: %year% %album% $if($eql(%_extension%,wav),(.wav),)
or
Format string: %year% %album% $if($eql(%_extension%,wav),(%_extension%),)
Addition:
My above suggestion adds a space after %album% in any case (also if there is no .wav extension).
Mp3tag/Microsoft don't let you create folders with a trailing space in its name.
Therefore you can't create a folder name like 2025 Waterloo ↑
You only get the trimmed folder name 2025 Waterloo
or extended folder name 2025 Waterloo (wav)
In the following suggestion from @ohrenkino the space will only be added if the extension .wav exists and (.wav) will be added.
The resulting folder name remains the same in both cases.