I can see what you mean with the fun defo haha, unlimited stuff don't always mean cheating or hand holding your right, but maybe a cool down would be better .as a nice middle ground, as its more so how quickly you can do it repeatedly and use it then how much you can use it also would be nice to have the shield evaporate on a timer after being inactive for practical and game play 🤔
Personally I have mmm trouble/style with a lot of ammo-preservation, and at most of games I prefer common and low level stuff or better infinity with more slow and accurate usage to progression.
This way, free shielding give me a big fun as free stuff itself with limited effective usage.
But agree with inactive disappearing, because it's funny when you reviewing level for secrets and found old shield used at level start. (But it's also harmless thing, and may be useful and funny enough at his current state. Simple timer can be not so good, but timer based on last damage time may be more difficult to implement and just over-featured)
My first reaction almost same as your, but over walkthrough i feel it's just already good enough.