LED chandelier lights permanently dim
Power supply is shot
G4 lamps are all 12V (least, all the ones I found).
All 20 lamps are behaving exactly the same. It can't be because of all the lamps being in series, since that would be 240V worth of lamps, and you're in 120V-land.  (that would be 2 series strings so only half would fail).   So cross that off the list.
What remains is there is a 12V power supply somewhere.  That is the common thread. Clearly, it is failing.