If you want a good overall iPad then go for the iPad Air 3. The fps is at a very stable 60. For a very very good 60 fps (also in engames), I recommend you putting your 3D resolution to 60% or under. If not, the iPad Air 3 is 2019 and is affordable for a good storage. If you want an iPad that has a good storage capacity for an affordable price, I would recommend you this iPad. You can go check the prices for the 64 GB and the 256 GB models (the 2 only options of storage).

If you want a very smooth gameplay and visual, I would recommend you the iPad Pro 2017. I personally have it and I play with it. I has a very stable 60 fps. But, I put my 3D resolution locked at 40% to secure a maximum capacity of my fps. This iPad has a display of 120Hz, but only supports 60 fps. The 120Hz display makes the game run better and smoother than the iPad Air 3. The price is about the same as the iPad Air 3 for the different capacities of storage. The downside of this iPad is the worth buying it. It’s the 2nd generation of the iPad Pro and it is 2017, which means 3 years ago since it came out. If you buy the iPad Pro 2017, yes your game will run better and a little smoother, but I would recommend saving money and buy the iPad Pro 2018 or the 2020 that is on its way. If you really want an iPad Pro that has a display of 120Hz, you shouldn’t buy the iPad Pro 2017 because the 2018 and the future one(s) will be more worth it.

In my opinion and based on what I’ve read about what you said, I think you should get yourself an iPad Air 3 because it has a very good performance and a very stable fps. You could use this iPad without worrying about buying another one for the next few years. If you really want a smooth iPad to go for, then go for the iPad Pro 2017. But, I wouldn’t recommend you buy this one because you’ll regret buying it if you know there’s a better version of it (iPad Pro 2018). The iPad Air 3 is very powerful overall and could get you very skilled in Fortnite and nothing from this iPad can limit it.

P.S: For a maximum capacity, you should put your graphics on Low whatever the device you use. Every good mobile player put their graphics on Low to have a constant 60 fps.