Holly cow, I've played so many times already. Very addictive!
If you're still planning for updating this game I'd like to suggest a small improvement for the upgrade selection: add a hold-to-choose feature. I.e. you choose an upgrade not just by clicking, but via holding left mouse until a meter fills up.
I've accidentally picked so many wrong upgrades when being super focused on fighting. It's a little annoying.
Another option could be press-a-specific-button-to-open-the-upgrade-menu. Maybe it's even better