The plastik part seems to be the resonator replacing it with a soft material that absorbs sound would be the easiest step.

However it might be better to disconect the speaker which should save lots of energy the peeper would consume otherwise.

I don't think apple checks for the speeker to be connected if the speaker breaks for example it would make the tag unusable resulting in more warrenty complains.

That kinda kills the whole purpose of the tag, which is to beep when you lose it. I guess you can still kinda find it with the UWB chip on your iPhone, but that's much harder than by sound.

Not if you want to use the tag to stalk someone.

Then just just a Tile or some other tracker.