Are you sure on that? With the extra water heater, you should be able to get the preheater to the same temperature as your water block exhaust temperature. With plate exchangers, you're still limited by the power of your loop. If you're running the hot water full out and pulling say 10lpm, you would need to dump ~700W of heat through the exchanger to raise the water 1 degree C. You would need a massive amount of plate exchangers and a lot of water in your cooling loop in order to raise the water temperature to close to the exhaust temperature. That sounds a lot more expensive than getting a free or very cheap water heater and some copper tubing, and much more likely to cause pressure issues on your hot water line.
Why?
Flat plate heat exchanger on the cold water line cost ~$50 and provides a massive Delta T to the cold water. It recovers most of the energy needed and is both cheap and simple. The payback period is measure in weeks. While one can do more remember hot water isn't that expensive maybe $300 per year so complex projects start cutting into your ROI% pretty quickly.
While there are ways to recover more of the heat I just don't see them as viable. Hacking up an old hot water heater of dubious quality, building your own radiator, running copper loop (not that cheap these days), jury rigging some water tight seals all to get maybe another 10% to 20% of annual energy budget? It doesn't seem worth it. I am going to keep it simple.