Good for you
exFat is a must, I guess, since you want to give him/them the ability to play games from SD (at least for the install).
As for the update with choidujour... Yes, 5.1.0 should enable them to play way more games. AutoRCM will also protect them from starting OFW and sending telemetry to Nintendo.
I personaly don't see a reason to go for SXOS as you get all games as NSP's and XCI's don't offer any advantages there, but thats a just me.
Just make sure to make a NAND backup, so they/you can restore it if anything goes wrong.
If possible, tell them to say offline to make extra sure they don't mess something up