A humble suggestion regarding buying skill upgrades.

I think a good idea would be this: if you double-tap a number 1-4, it upgrades that skill once. If you triple-tap, it upgrades it twice (if possible). I think this would help keep me in the action and from having to retreat just to buy skills (without being sniped).

Or is there already a quick way of doing this without bringing up the skill screen?

Using the numpad I can already upgrade really fast. I frequently upgrade in combat, and I haven't yet died while upgrading (don't have the tag)

Num+ brings it up, num8 upgrades yellow, 6 upgrades red, 2 upgrades passive, 4 upgrades blue.

Just a real quick "+2+" upgrades the passive skill

Yeah you have to take your hand off the mouse for a second, but you can't aim while upgrading anyway. Plus movement is more important than aim for dodging.