Delamination repair

It really depends on how bad the delamination is and how much damage there is to the substructure. There are a number of thin epoxies like water glass or git rot. You can also just add more solvent to epoxy or polyester resin to thin it down. The solvent will flash off leaving a good bond. To use epoxy or polyester resin the substrate needs to be completely dry which may prove difficult. I removed the window on mine and put wedges between the FG and the wood and foam and parked that side in the sun for 5 days. That still was not enough to dry the substrate. I also add acetone to replace the moisture which helped but still did not get all the water out. I ended up using polyurethane resin because it will react to moisture and cause a foaming action which will weaken the bond somewhat. If you have a lot of rot underneath I like your diamond plate approach, The .050-.125 plate will be stronger than .030 FG outer skin especially if you mechanically fasted to the Al framing. You can also use thicker FG skins or smooth Al skin to better match what you have now. Either way it will be a big project and good luck to you either way you go.