I haven't tried but I have a bit of experience on the Atlantis so here goes.
Of primary concern as to nicotine, commonly you find people who
vape drippers and other sub-ohm equipment using considerably lower nicotine concentrations. I was personally at 6mg but down to 3 now and find it suits all my needs adequately.
As for viscosity and how that affects the vape. Well, it's winter and if you spend much time where ambient temperature is 60 deg or lower you need to have a lower percentage of VG simply because the Atlantis cannot wick the thicker liquids as fast as you can vaporize them...ergo, dry hits. I found this out the hard way and yes, here in SC we've had those cooler nights but winter...she's a comin! But, in warmer weather such as we were having when I bought my Altantis I could easily vape 80/20 VG/PG. Now I'm 60/40...going as high VG as I can because that's what I like. Knowing how OK can get some pretty darn cold weather, you may want to do 50/50 until the summer. Not sure what Firebrand ratios are but most liquids I've seen marketed for sub-ohm use have the dripper in mind, and consequently higher VG.
Just one man's opinion. But man, you're gonna love that
tank! Best o' luck.