I think there might be some bias in the above haha.
My opinion on the right firmware is the one that does what you need it to do. If they all do what you want, go with the one that has the biggest community (support) behind it, will be easier to get help for.

When I saw the tittle of the thread I thought you were scrapping an old flashforge and using the parts for a new machine. Upgrades for an existing machine? Experiment, try bowden, direct, geared or ungeared extruders, removable beds, auto leveling. Go nuts and have fun.