Amazing!
As for putting damage upgrades in front of the Tanker--I'd suggest at least doing it in parallel rather than as a requirement to unlock them. Otherwise you're essentially lowering their health, which kind of negates their special property!
I think giving the player CHOICES on how they want to progress is the fun bit. Even something like exposing enemy stats instead of suggesting a strategy might put more power into the players' hands to make their own choice and live with the consequences. If there's a big number in the HP stat, with maybe a description like "defensive shapes" or something rather than "go buy damage before buying this!", it will make the player feel more clever when they decide to prioritize the damage upgrade first!
I think making enemy upgrades cheap to encourage progression is a great idea. I didn't notice that, but I think I enjoyed the game more because of it!
Check out the game Nodebuster if you haven't -- it has a unique "combat" system and a nice progression. The shop has a good system to represent the bazillion states an upgrade item can be in.