Hi, I have experience with arduino and raspberry pi, I have done lot's of projects with both.
I saw that you have an arduino code but it's not completed or maybe you just erased some functions.
By that file I don't understand if you want to use just the raspberry pi or both arduino and raspberry pi.
My advice is to use only raspberry pi, is more than enough and perfectly capable.
If you are interested in my offer I will need more details, such as screen type and dimensions, GUI layout, images or just a sketch and a few details on what hardware you want to use to control the pumps.
If you want I can advise you on what will be best for you.
Feel free to ask any questions.
Best regards,
Nélio Oliveira