Hello, I have been a little bit busy lately and couldn’t respond earlier.
I do really hate to say this, knowing that you have already made your mind in this topis, but unfortunately RuBisCo is already being used in the game. RuBisCo is a carbon-fixing protein, this means that it uses the CO2 on the enviroment to generate Glucose/Glucolitic metabolites wasting ATP. In other words, the Calvin cycle - Wikipedia, which is indeed part of the photosynthesis so actually RuBisCo is already being “used” when the player gets the Thylakoid organelle (or the Chloroplast).
Whenever I have more time I’ll try to research more on this topic, though I really believe that Gluconeogenesis + the glucoxylate cycle is the only metabolic pathway that serves this purpose.