You seem to be confused with what you want to do with your second switch.
You say it's for online play, which means you should keep it updated to latest FW, and hack-free.
But you've never set it up for connecting to wifi, and seem to be hesitant to update it (dare I say, you might even be considering preserving the fuses?)
Like you SAY it's for online play, BUT you ALSO want to keep your downgrade options open...
I don't blame you though, I'm the same way
My final decision was to upgrade via OFW, BUT preserve the fuses (by inserting a 100% reliable jig, like the SX pro jig, when it is about to restart after updating, and holding vol+ when it does, THEN booting into Hekate. I made a 5.1 clean NAND backup as well, and had NEVER gone online since that backup was made, so that if I restore it, my logs will have no "gaps/rollbacks" that Nintendo will notice).
So now I have preserved my fuses without ever installing Homebrew (i.e. using ChoiNX), I have a clean NAND backup of 5.1 I can go back to IF I ever want to play online, AND a clean NAND backup of 4.1 (made when the Switch was new), that I can boot because I still have my original fuses intact.
I DID have to enable AutoRCM after making the 5.1 backup so that I don't accidentally burn my fuses.
And when 6.0 rolls out, I plan to disable AutoRCM, restore my 5.1 backup, then boot into OFW using Hekate, update to 6.0, insert the jig and hold vol+ as it restarts after, and make a 6.0 backup as well as delete my wifi settings again. So I have a 6.0 clean NAND backup I can always restore, OR a 5.1 backup I can restore while CFWs catch up to 6.0, AND also all my fuses.
The ONLY thing that can ever screw me is if Horizon starts detecting fuse count mismatch.