Others have correctly pointed out... Screen size above all else. I think you will be fine with Android as long as they have the app you plan on using for reading charts. You could also use one of the older iPad Pro models to reduce the price of the purchase. Ultimately you should not need a ton of processing power for reading charts. Screen brightness may matter depending on environment you plan on playing in. Even the brightest iPads are not a great reading experience in direct sunlight.
I have tried reading charts on iPhone 12 Pro Max, 2018 iPad Pro 11", and 2020 iPad Pro 12.9". Even though I am in my 30s, the 12.9" is much more usable than the smaller screens. The 6.7" on the iPhone was barely doable for me (noting the time I tried it was for a quick lookup and not an extended session).
I have tried reading charts on iPhone 12 Pro Max, 2018 iPad Pro 11", and 2020 iPad Pro 12.9". Even though I am in my 30s, the 12.9" is much more usable than the smaller screens. The 6.7" on the iPhone was barely doable for me (noting the time I tried it was for a quick lookup and not an extended session).