Doshima on discord confirm this update is in ren'py. Shaman write about items and money system how work in ren'py so i guess it's on ren'py. Thank god atleast in will be less painful. I hope...
I think there is nothing wrong with RPGM if this was a completely free roam sandbox RPG, but instead it is a Linear Visual Novel (and RPGM can make VNs, dev just chose RPG gameplay) (and linear as in, once you choose a path you can't change afaik).
The idea of the game was just not compatible with the gameplay, which made it feel slower than it should be. Way too many pauses, useless repetitive walking segments, slow transitions, slow minigames, likelihood of random bugs.
I haven't played the unofficial port so when the update comes out it will be my first time trying out the engine change.