Congrats on your accomplishment. You are fortunate to find a first device that got you off Smokes. You are now at the second step, which I think is the hardest. The desire to advance a little. This is where everyone gets in a battle of finances, too many choices, and urge to succumb to shinyitis.
First, you definitely need a second device. Either for a backup, to add in a rotation, or as a primary if you move your Smokstik to backup.
If you aren’t ready for batteries, chargers, etc. there are a couple of popular internal battery mods that are versatile and are great mid level units. First is the Aspire Zelos kit that comes with the Nautilus 2 tank. The second is the iStick 30w with the Nautilus 2 tank. You buy those separately. Many members here have used those two, some never moving beyond that.
If you want to move up to external batteries and charger, I’m not going to venture a guess. It really will depend on your style of vaping, MTL or DTL. There are countless choices.
I am a dedicated, tight draw, MTL guy. Started with junk, moved to the fixed wattage Endura T22 kit. Tomorrow will be 6 months Vaping, and my current rotation is multiple Provaris, an SVD 2.0, and sometimes the old Endura mod. Tanks are Nautilus 2’s, Kabuki clones, and T22’sI don’t see myself changing what I have. But that’s me. However, there are other 6 month vapers that are using multiple battery mods and sub ohm rebuildables. Some people use both.
Good luck. You’ll figure it out.