I was curious It probably is wasteful, but tanks don't go bad, do they?
I bought one Protank and I like it, so I ordered three more. The main reason for this is I didn't want to worry about my juice eating through plastic tanks. I admit I didn't do enough research about metal tanks, however, what's done is done.
Tanks don't go bad but they don't last forever. I bought 2 Vision Mini Novas and 12 replacement heads - thinking that when the replacement heads were used up I could start rebuilding them myself.
However, the connector on the bottom of the tanks loosen up over time and now my concern is that my tanks will last long enough for me to use all the replacement heads I bought.
7 tanks to start off with might be a little excessive .. but tanks are great and I have no doubt you'll eventually use them all. I don't really like to buy them up, though - because new stuff is always coming out and they stop selling replacement heads for the old stuff. By the time your last tank dies, replacement heads could be hard to find - of course, you can rebuild them yourself.
You probably bought 2 more batteries than you really need - but they only last about a year and then should be replaced - so you've got 2 years worth of batteries . .no big deal. You probably overbought e-juice . .if you bought 9 bottles chances are you won't like them all and some will go unused . .but everybody does that.
So I would say you're right on track - off to a good start.