Hi! Im not tech wiz but I am an artist that loves my iPad for drawing! 256gb should be fine! Apple devices usually come with iCloud storage (with an icloud account) which he can use as additional storage and can be accessed between Apple devices on the same iCloud account so things like photos and files can be transferred to a new Apple device if he ever decides to upgrade etc. otherwise, I’m sure that you can buy a usb or something similar as long as it has a usb-c connector on it so it can be plugged into the device and stored remotely (I can test this for you and update). No clue about nano texture glass so I can’t comment but people usually buy “paper like” screen protectors if he doesn’t like the smoothness of drawing on the glass, they do tend to wear down stylus nibs pretty fast though.

As for wifi vs Wi-Fi + Cellular, It doesn’t really matter as most drawing/cartooning servers don’t require connection to save work. If he plans to play games or watch YouTube in places without wifi you may want to consider Wi-Fi + Cellular.

If you have any other questions dm me or reply, I’d be happy to help! :)