I think that a good shortcut could be had by eliminating the step of isolating the ferric oxolate. You could simply add oxolic acid, ferric nitrate, and the right amount of sodium hydroxide to your working solution. As long as the residual sodium nitrate doesn't interfere with the developing, you should be in good shape.
As others have mentioned you are making a platinum print, not ferric oxalate. I see no reason to isolate it.