Holy $%$% 21k. I work with a guy who had an entire pool done for 25k. Lol. This is an amazing pond though. Love the layout. The round rock. And the pavers. Great so far.
Any luck getting a lower price on the polyurea? Is the builder going to help you out with the leaking issue?
Sorry for your pain. Is the leak limited to where the wires are? What is the % of failure so far?
Sound like you have already gone thru all the logical options. Liner or Polyuria are probably the safest long term bet. 21K sounds steep but it probably has to do with your ZIP code.
If the leak is limited to the tunnels created by rusted wires, one wild idea I have is to apply air suction on one side of the wall while applying multiple layers of epoxy on the other side. The hope is to get enough epoxy into the tunnels so they are effectively sealed. This is a long shot and may not even be practical.
Bump for updates
....."Never argue with stupid people, they will drag you down to their level and then beat you with experience.".....Mark Twain