Easiest method.
SXOS on Emunand permanent aeroplane+ stealth mode, don't use autoRCM. Can "double wrap" by using 90DNS if you want too.
Keep Sysnand clean 100%
When want to go online, power off, remove sd card, boot OFW
Play your games via legit carts/eshop installed on sysnand or another SD card.
My OFW is still on 4.0.1 so I think even if Nintendo hid some telemetry in their firmwares, it'd be on 5.x upwards.