Use Checkpoint to backup your saves to your SD card. Restore your NAND, play smash online, and when you're ready to stop playing online and start using CFW again, then you can use Checkpoint to restore your save.
If you wanna play risky, I've played online with Hekate for months and never got banned, and I did a lot of save backups and restoring using Checkpoint, but I haven't gone online since they released the Nintendo Online Subscription. I'd imagine that using CFW to restore your saves and play online is a lot more risky with 6.2.
Depends on how much playing online is worth to you.