Low light illumination on ceiling fan
There are voltage limiters in ceiling fans these are to prevent people from installing lamps that are a higher wattage than the fixture is listed for and overheating the fixture.
They were first mandated around 05 so this may be your issue.
It looks like a black square and has 2-3 wires as these fail they do get dimmer or go completely out.
I looked some up last week on amazon around 8$ for a replacement a 3 wire can replace a 2 wire but you have to connect the white to the neutral the 2wire is a direct replacement if you have a 2 wire but both are available the 3 wire last longer.