Apply the patch to a clean .iso. (so not an XG one, but an XD one)
What I did was keep a clean iso in a separate folder and copy it each time there was a new patch and applied the patch to that copy.
Of course ! Try to export your save first, in case someone would be able to help you out. Just click on "tools" then "Gamecube Memory Card Manager". Image in spoilers below!
Really not sure if that will work, but maybe you could try to load a savestate? Would be interesting to know what would happen in that case.
If you still wanna keep a regular save and not using save states, you can make a new file on v1.1.8, save, load state (if it works) and then the game should let you resave normally. Once again, if loading a state works... Really don't know how that stuff works since all I knew was that there were some differences in the saves because of the patches, never heard about incompatible saves because of it.