This tutorial (for custom planet textures) helped me a lot. I used a greyscale map of Pontus so that the proper elevations, sea level, etc. can be set up.
As for creating the star and planet, it involves exporting some random star's/planet's file, editing it with Notepad++, and putting them into custom folders. I assumed the star that Pontus orbits around is basically like the Sun.
Like 1 week, or 1 month or longer?
My game really slows down quite badly after a while... my phone is not great in terms of CPU, but it's definitely above average for android, and I have 6gb of ram on it. https://www.gsmarena.com/xiaomi_mi_max_3-8963.php
Just download and use an Android emulator, such as Bluestacks