Full disclosure: I haven't gotten below 2:10 yet, but I have some thoughts. First off, this is a great idea, and the execution is solid. I like that since the technical skill ceiling is kind of low, it's more about making the right choices to make a build that both survives the immediate threats and has a chance to win later on.
That said, I have a couple thoughts on balancing, since the game is punishingly difficult right now. I think I played for a good 30 minutes and couldn't get more than 3/5ths of the way through.
As you say, movement speed is not useful. I would consider cutting it entirely. It really sucks when that gets rolled alongside an extra life I don't need. I end up missing out on an entire level worth of scaling, which can be brutal at the start. Getting a gun upgrade vs taking an extra life at level one is the difference between a one minute run and a three minute run.
It would be nice to have a defensive upgrade besides extra lives though. Maybe an increase to how long your invincibility lasts?
Other thoughts: the enemy clumping, especially the blobs, is a little overkill. They start out far apart, then end up really clumped together, which makes sense for scaling, but then it switches to blue blobs, which are harder to kill, and they start out clumped. It's a huge difficulty spike at the same time the big demon guy appears. Maybe a minimum amount of space between each blob? Just a couple pixels so they're not all directly on top of each other.
I don't get why barbed wire is half damage since you have to risk being close to the enemies to do anything with it. There's probably a strat using invincibility that I couldn't pull off, but I don't know if that'd be gamebreaking at full damage.
Overall, I liked it a lot. I played it for half an hour and wrote this whole thing. I just wish I had more of an ability to affect my runs. Seems like it's mostly just a matter of high-rolling gun>atkspd>dam a bunch in the beginning.
Okay, so I wrote this whole thing out then went back to play a few more runs and ended up winning. I think most of my points still stand, but I will say it's a nice touch that you can start over with all your upgrades and get that power fantasy of laying waste to everything. So yeah, probably spent like 45 minutes on this total. It's solid.