Please define "properly"; No one really knows how Big N is able to detect the CFW... your statement is more of an opinion ------> "When done properly Nintendo has no way of knowing that you've been running CFW"
That's where you're wrong, it's certainly not an "opinion." No, we don't have everything figured out, but we know what will obviously get you banned (going online with cheats enabled, certain edited save files, or illegit NSPs installed.) Atmosphere does manage to disable the telemetry though.
But just booting RCM mode doesn't normally leave any traces on your console. There are certain things you can do without affecting your NAND, like booting Linux, Android, or Lakka. If you boot Hekate, you can make a NAND backup, and Nintendo literally has NO way of knowing you used RCM. Then, you can use that NAND backup to create an emuMMC partition on your microSD. If you use that emuMMC partition with Atmosphere, no traces of it will be left on your NAND. After that point, the important thing is to keep your emuMMC offline.Clean sysNAND, dirty emuNAND is a good method used by a good number of people here to keep from getting banned.
There are some extra things I should mention considering I said
properly:
AutoRCM: AutoRCM is a technique people use that temporarily "bricks" your console by changing BOOT0. This causes it to boot into RCM without a jig. In order to boot stock with AutoRCM enabled, you need to use Hekate or another custom bootloader. Currently we have NO evidence that Nintendo looks at this for banning purposes, but that could always change with an update. So if you want to be 100% safe, I'd suggest not using AutoRCM.
Separate microSD Card: Once again, we currently have NO evidence that Nintendo bans users based on the contents of their microSD card. But just to be on the safe side, I use a separate microSD card for my emuNAND and use a different one for when I boot stock.
Going online with emuNAND: Many people using "clean sysNAND, dirty emuNAND" will use Incognito, 90DNS, or both, to go online while in emuNAND. And currently this seems to be ban-proof. But once again, with an update, Nintendo could decide to rely on a static IP rather than DNS, or they could check for linked NNIDs on the console and ban your sysNAND based on that rather than relying on the PRODINFO that Incognito removes. So offline emuNAND is 100% safe, but it's still very unlikely you'd be banned if you used one of these other methods.
So no, it's not an "opinion", there are safe, ban-proof methods of using CFW on your Switch without buying a second Switch.